A one-way clutch comprising an outer race provided with a pocket and having a groove portion formed at an inner diameter side of an axial one end face of the outer race, an inner race arranged coaxially with the outer race, a roller disposed in the pocket and adapted to transmit torque between the outer race and the inner race, a cage having a cylindrical portion with a window and a flange portion extending from an axial one end face of the cylindrical portion in an outer diameter direction, a projection provided on the cylindrical portion, a spring disposed in the pocket between the outer race and the roller and adapted to bias the roller toward an engagement direction with respect to the cam surface, and a side plate secured to abut against the axial one end face of the outer race and provided at its inner diameter portion with spline grooves, and wherein the cage is prevented from being rotated relative to the outer race by fitting the projection in the groove portion.