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A valve includes a valve element; a valve seat; a coil spring; and a spring bearing portion. The coil spring, provided between the valve element and the spring bearing portion, presses the valve element toward the valve seat. An effective upper winding end of the coil spring and an effective lower w

A valve includes a valve element; a valve seat; a coil spring; and a spring bearing portion. The coil spring, provided between the valve element and the spring bearing portion, presses the valve element toward the valve seat. An effective upper winding end of the coil spring and an effective lower winding end of the coil spring are offset from each other by 180 degrees in a plan view.

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What is claimed is: 1. A valve comprising: a valve element; a valve seat; a coil spring; and a spring bearing portion, wherein the coil spring, provided between the valve element and the spring bearing portion, presses the valve element toward the valve seat, wherein the coil spring has an effectiv

What is claimed is: 1. A valve comprising: a valve element; a valve seat; a coil spring; and a spring bearing portion, wherein the coil spring, provided between the valve element and the spring bearing portion, presses the valve element toward the valve seat, wherein the coil spring has an effective winding region that bends when an axial load is applied to the coil spring, the effective winding region being between an effective upper winding end, which is a point in an upper portion of the coil spring at which an axial direction rigidity of the upper portion of the coil spring is largest, and an effective lower winding end, which is a point in a lower portion of the coil spring at which the axial direction rigidity of the lower portion of the coil spring is largest, and the effective upper winding end of the coil spring, and the effective lower winding end of the coil spring being offset from each other by 180 degrees in a plan view. 2. The valve according to claim 1, wherein: when the valve element is moved in a movement range, adjacent winding portions of the coil spring do not contact each other. 3. The valve according to claim 1, wherein: the upper portion of the coil spring includes a first winding portion that contacts the valve element, a second winding portion that is continuous with the first winding portion; and the lower portion of the coil spring includes a third winding portion that contacts the spring bearing portion, and a fourth winding portion that is continuous with the third winding portion; and when the valve element is moved in the movement range, positions of the effective upper winding end and the effective lower winding end are maintained by disposing the first winding portion and the second winding portion such that the first winding portion and the second winding portion do not contact each other, and by disposing the third winding portion, and the fourth winding portion such that the third winding portion and the fourth winding portion do not contact each other. 4. The valve according to claim 3, further comprising a first spacer disposed between the first winding portion and the second winding portion. 5. The valve according to claim 3, further comprising a second spacer disposed between the third winding portion and the fourth winding portion. 6. The valve according to claim 1, wherein: the upper portion of the coil spring includes a first winding portion that contacts the valve element, a second winding portion that is continuous with the first winding portion; and the lower portion includes a third winding portion that contacts the spring bearing portion, and a fourth winding portion that is continuous with the third winding portion; and the first winding portion and the second winding portion are disposed such that a clearance between the first winding portion and the second winding portion is maintained when the valve element is moved in a movement range, and the third winding portion and the fourth winding portion are disposed such that a clearance between the third winding portion and the fourth winding portion is maintained when the valve element is moved in the movement range. 7. The valve according to claim 6, further comprising a first spacer disposed between the first winding portion and the second winding portion. 8. The valve according to claim 6, further comprising a second spacer disposed between the third winding portion and the fourth winding portion. 9. The valve according to claim 1, wherein: the coil spring is an open-end constant-pitch spring that is attached to an upper support member at an upper-end winding portion of the open-end constant-pitch spring, and that is attached to a lower support member at a lower-end winding portion of the open-end constant-pitch spring; the upper support member and the lower support member are opposite to each other through the open-end constant-pitch spring; a first spiral surface is formed on a surface of the upper support member, and a corresponding spiral surface is formed on a surface of the lower support member, the surface of the upper support member and the surface of the lower support member being opposite to each other; a side face formed in the upper support member extends from a start portion of the first spiral surface of the upper support member in an axial direction of the coil spring by a distance equal to the pitch of the open-end constant-pitch spring; a corresponding side face formed in the lower support member extends from a start portion of the corresponding first spiral surface of the lower support member in the axial direction of the coil spring by a distance equal to the pitch of the open-end constant-pitch spring; and the first spiral surface of the upper support member receives the upper-end winding portion of the open-end constant-pitch spring, and the corresponding first spiral surface of the lower support member receives the lower-end winding portion of the open-end constant-pitch spring. 10. The valve according to claim 9, wherein: a second spiral surface is formed in the upper support member that extends parallel with the corresponding first spiral surface of the upper support member, and a distance shorter than the pitch of the open-end constant-pitch spring separates the second spiral surface and the first spiral surface; and a corresponding second spiral surface is formed in the lower support member that extends parallel with the corresponding first spiral surface of the lower support member, and the corresponding second spiral surface is separated from the lower spiral surface by a distance shorter than the pitch of the open-end constant-pitch spring. 11. The valve according to claim 1, where the coil spring is an open-end constant-pitch spring that has a rectangular cross section that has a long side extending in an axial direction of the coil spring, and a short side extending in a radial direction of the coil spring, and a flat surface portion is formed in each of the upper-end winding portion and the lower-end winding portion. 12. The valve according to claim 1, wherein the coil spring is formed by forming a spiral opening that has a constant pitch on an outer periphery of a spring steel pipe. 13. The valve according to claim 1, further comprising: a case, in which the valve element is disposed; and a fluid passage formed in the case through which fluid is delivered from a tank by a pump, wherein the valve element contacts the valve seat to close the passage when a pressure of the fluid in the passage is below a predetermined value, and the valve is moved away from the valve seat to open the passage when the pressure of the fluid is above the predetermined value. 14. A vehicle fuel system, comprising: the valve according to claim 1. 15. The valve according to claim 1, wherein a diameter of the coil spring is constant from the effective upper winding end of the coil spring to the effective lower winding end of the coil spring. 16. The valve according to claim 1, wherein a diameter of a first winding portion of the coil spring, which includes the effective upper winding end, is approximately the same as a diameter of a second winding portion of the coil spring, which includes the effective lower winding end.