A clutch mechanism comprises a planetary gear unit including input transmission, output transmission, and transmission stopping members. Torque entering the input transmission member is outputted downstream of the planetary gear unit by the output transmission member when the transmission stopping member is fixed and is not outputted downstream when the transmission stopping member is unfixed and entering a rotatable state. A rotation prohibiting target has an engaging target and is integral with the transmission stopping member. A rotation prohibiting member has a supporting shaft, a swingable portion at its one end and an engaging portion at its other end to engage the engaging target to enable the transmission stopping member to exert the stopping function. The engaging portion disengages the engaging target to bring the transmission stopping member into an unfixed state. A damper unit absorbs vibration caused in the rotation prohibiting member during a switching operation.