Since one end of a spring (22) is received by a planar portion of a spring seat (24) provided on any one of an outer plate (14) and an inner plate (12), it is possible to support the spring with a planar member (28) without machining a bottom wall face of the outer plate (14) into a planar shape. Since the spring seat (24) has seat guide portions (32a, 32b) that contact the outer plate (14), it is possible to suppress the fluctuation of the spring seat (24) and the fluctuation of the spring (22) by the seat guide portions (32a, 32b), so it is possible to reliably prevent interference of the spring (22) with its peripheral portion. Therefore, a load on the spring (22) is reduced, so it is possible to improve the durability of a one-way clutch (10).