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A fixing device includes a tubular member, a nip member, a backup member, and a bias member. The tubular member has an outer peripheral surface, an inner peripheral surface defining an internal space, and an axis defining an axis direction. The nip member is in contact with the inner peripheral surf

A fixing device includes a tubular member, a nip member, a backup member, and a bias member. The tubular member has an outer peripheral surface, an inner peripheral surface defining an internal space, and an axis defining an axis direction. The nip member is in contact with the inner peripheral surface. The backup member is opposed to the nip member and in contact with the outer peripheral surface to form a nip region for nipping a sheet conveyed in a conveying direction orthogonal to the axis direction. The backup member is configured to move in the conveying direction and the tubular member is configured to circularly move around the axis in accordance with the movement of the backup member due to a friction force generated between the tubular member and the backup member. The bias member protrudes toward the backup member through a bias region.

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1. A fixing device comprising: a flexible tubular member;a nip member having a contact surface configured to be in contact with an inner peripheral surface of the flexible tubular member; anda backup member, the backup member and the contact surface of the nip member being configured to pinch the fl

1. A fixing device comprising: a flexible tubular member;a nip member having a contact surface configured to be in contact with an inner peripheral surface of the flexible tubular member; anda backup member, the backup member and the contact surface of the nip member being configured to pinch the flexible tubular member therebetween,wherein the contact surface includes: a first portion, the first portion and the backup member being configured to pinch the flexible tubular member therebetween; anda second portion extending from the first portion and protruding from the first portion toward the backup member, the second portion and the backup member being configured to pinch the flexible tubular member therebetween,wherein the flexible tubular member is configured to move in a moving direction between the nip member and the backup member, andwherein a dimension of the second portion in the moving direction is greater than a dimension of the second portion in a direction parallel to an axis of the flexible tubular member. 2. The fixing device according to claim 1, wherein the second portion has a convex shape. 3. The fixing device according to claim 2, wherein the second portion has a convex shape when viewed from a direction parallel to an axis of the flexible tubular member. 4. The fixing device according to claim 1, wherein the nip member includes a metal plate. 5. The fixing device according to claim 4, wherein the nip member is the metal plate. 6. The fixing device according to claim 5, further comprising a heater spaced apart from the nip member. 7. A fixing device comprising: a flexible tubular member;a nip plate having a contact surface that is configured to be in contact with an inner peripheral surface; anda backup roller, the backup roller and the contact surface being configured to pinch the flexible tubular member,wherein the contact surface includes: a first portion, the first portion and the backup roller being configured to pinch the flexible tubular member therebetween;a second portion extending from the first portion and protruding from the first portion toward the backup roller, the second portion and the backup roller being configured to pinch the flexible tubular member therebetween; anda third portion extending from the first portion and protruding from the first portion toward the backup roller, the third portion and the backup roller being configured to pinch the flexible tubular member therebetween, the third portion being spaced away from the second portion in a direction parallel to an axis of the tubular member. 8. The fixing device according to claim 7, wherein the second portion is parallel to the third portion. 9. The fixing device according to claim 7, wherein the flexible tubular member is configured to move in a moving direction between the nip plate and the backup roller, wherein the second portion is parallel to the moving direction, and the third portion is parallel to the moving direction. 10. The fixing device according to claim 7, wherein the second portion has a convex shape, and the third portion has a convex shape. 11. The fixing device according to claim 10, wherein the second portion has a convex shape when viewed from a direction parallel to an axis of the flexible tubular member, and the third portion has a convex shape when viewed from a direction parallel to the axis of the flexible tubular member. 12. The fixing device according to claim 7, wherein the flexible tubular member is configured to move in a moving direction between the nip plate and the backup roller, wherein a dimension of the second portion in the moving direction is greater than a dimension of the second portion in a direction parallel to the axis of the flexible tubular member, and a dimension of the third portion in the moving direction is greater than a dimension of the second portion in a direction parallel to the axis of the flexible tubular member. 13. The fixing device according to claim 7, wherein the nip plate includes a metal plate. 14. The fixing device according to claim 13, wherein the nip plate is the metal plate. 15. The fixing device according to claim 7, further comprising a heater spaced apart from the nip plate. 16. The fixing device according to claim 7, wherein the second portion of the contact surface of the nip plate is opposite to the third portion relative to an imaginary plane that passes a center of the nip plate in a direction parallel to the axis of the flexible tubular member. 17. The fixing device according to claim 16, wherein the flexible tubular member and the backup roller are configured to convey a sheet therebetween, and wherein a distance between the second portion and the third portion is greater than a dimension of the sheet that is to be conveyed between the flexible tubular member and the backup roller. 18. The fixing device according to claim 17, wherein the flexible tubular member includes a film. 19. The fixing device according to claim 7, wherein the contact surface of the nip plate is configured to contact the inner peripheral surface of the flexible tubular member via grease. 20. The fixing device according to claim 1, wherein the contact surface of the nip member is configured to contact the inner peripheral surface of the flexible tubular member via grease. 21. A fixing device comprising: a flexible tubular member;a nip member having a contact surface that is configured to be in contact with an inner peripheral surface; anda backup roller, the backup roller and the contact surface being configured to pinch the flexible tubular member,wherein the contact surface has: a first portion, the first portion and the backup roller being configured to pinch the flexible tubular member therebetween;a second portion extending from the first portion and protruding from the first portion toward the backup roller, the second portion and the backup roller being configured to pinch the flexible tubular member therebetween; anda third portion extending from the first portion and protruding from the first portion toward the backup roller, the third portion and the backup roller being configured to pinch the flexible tubular member therebetween, the third portion of the contact surface of the nip member being opposite to the second portion relative to an imaginary plane that passes a center of the nip member in a direction parallel to an axis of the flexible tubular member, the imaginary plane being perpendicular to the axis of the flexible tubular member. 22. The fixing device according to claim 21, wherein the flexible tubular member and the backup roller are configured to convey a sheet therebetween, and wherein a distance between the second portion and the third portion is greater than a dimension of the sheet that is to be conveyed between the flexible tubular member and the backup roller. 23. The fixing device according to claim 22, wherein the contact surface of the nip member is configured to contact the inner peripheral surface of the flexible tubular member via grease.