To prevent a transmitted gear from withholding a power transmission gear by the rotation when the power transmission gear is about to separate from the transmitted gear.

A gear device 8 includes a first planetary gear 95 performing sun-and-planet motion around a driving gear 94 and carries out ON/OFF switching of power transmission to the transmitted gear 75b by the sun-and-planet motion of the first planetary gear 95. The gear device 8 is equipped with an auxiliary gear 98 performing the sun-and-planet motion around the transmitted gear 75b. The auxiliary gear 98 is separated from the first planetary gear 95 when the first planetary gear 95 is engaged with the transmitted gear 75b to transmit the power. When the first planetary gear 95 is about to be separated from the transmitted gear 75b according to switching of the rotational direction, the auxiliary gear 98 is engaged with the first planetary gear 95 to give a force in a direction separated from the transmitted gear 75b to the first planetary gear 95.