In a clutch roller (13) to be provided in a unidirectional rotation-type clutch device that is fitted through a helical spline into a drive shaft, which is rotated upon the action of driving force of a motor part, and is constructed so that the power of the motor driving force is transmitted, but on the other hand, the power of the engine driving force upon the engine start is not transmitted, the thermal influence, in the case where an excessive load above a usual overrun is applied from the engine side, is reduced to enhance the durability. The carbonitriding treatment of the clutch roller causes penetration and diffusion of carbon and nitrogen to form, on the clutch roller (13), a compound film layer which can reduce thermal influence to improve the durability.