A motor actuator and an opening/closing device. In the driving force transmission mechanism (3) of a motor type damper device, when a small AC synchronous motor (2) is rotated in one direction and the tooth-lacking-gear part (462) of a gear (46) with tooth-lacking gear is engaged with the first rack part (47) of a rack member (8), the rack member (8) is displaced upward to operate a baffle in the opening direction, and when the tooth-lacking-gear part (462) is engaged with the second rack part (48) of the rack member (8), the rack member (8) is displaced downward to operate the baffle in the closing direction. Even when the baffle and the rack member (8) are operated in both directions, the gear (46) with tooth-lacking gear may be rotated in one direction only and the small AC synchronous motor (2) must not be reversely rotated. As a result, the structure of a control circuit for the motor type damper device can be simplified.