A sun gear member, planetary gear members, a first ring gear member, and a second ring gear member together form a paradox gear mechanism. A clutch mechanism is disposed between an input gear member and the first ring gear member. The clutch mechanism rotates the first ring gear member, when the input gear member rotates. The clutch mechanism disconnects transmission of the power to the input gear member when the first ring gear member rotates. The sun gear member and the first ring gear member are individual input elements. When one of the sun gear member and the first ring gear member exclusively rotates, the second ring gear member rotates in accordance with the rotation. The sun gear transmits power more reliably without increasing manufacturing cost and without thwarting downsizing of the product.