A rotation actuator comprising: first fixed member and second fixed member fixed to each other across a predetermined gap in a center axis direction; a rotating shaft that passes through the first fixed member in the center axis direction, a leading-end part of the rotating shaft extending to the second fixed member; a first bearing mounted between the first fixed member and the rotating shaft, the first bearing rotatably supporting the rotating shaft on the first fixed member; a second bearing mounted between the second fixed member and the leading-end part of the rotating shaft, the second bearing rotatably supporting the rotating shaft on the second fixed member; a motor rotor fixed to a trailing-end part of the rotating shaft, the trailing-end part protruding toward a side opposite to the second fixed member; a motor stator fixed to a portion of the first fixed member located on an opposite side from the second fixed member, the motor stator surrounding an external periphery of the motor rotor across a fixed gap; a rotation output member for surrounding an external periphery of the second fixed member; a third bearing mounted between the second fixed member and the rotation output member, the third bearing rotatably supporting the rotation output member on the second fixed member; and a reduction gear mechanism incorporated between the first fixed member and the second fixed member, the reduction gear mechanism reducing rotation of the rotating shaft and transmitting reduced rotation to the rotation output member; and a sliding bearing mounted between the rotation output member and the first fixed member so as to support the rotation output member in a rotatable manner on the first fixed member in the center axis direction. first fixed member and second fixed member fixed to each other across a predetermined gap in a center axis direction; a rotating shaft that passes through the first fixed member in the center axis direction, a leading-end part of the rotating shaft extending to the second fixed member; a first bearing mounted between the first fixed member and the rotating shaft, the first bearing rotatably supporting the rotating shaft on the first fixed member; a second bearing mounted between the second fixed member and the leading-end part of the rotating shaft, the second bearing rotatably supporting the rotating shaft on the second fixed member; a motor rotor fixed to a trailing-end part of the rotating shaft, the trailing-end part protruding toward a side opposite to the second fixed member; a motor stator fixed to a portion of the first fixed member located on an opposite side from the second fixed member, the motor stator surrounding an external periphery of the motor rotor across a fixed gap; a rotation output member for surrounding an external periphery of the second fixed member; a third bearing mounted between the second fixed member and the rotation output member, the third bearing rotatably supporting the rotation output member on the second fixed member; and a reduction gear mechanism incorporated between the first fixed member and the second fixed member, the reduction gear mechanism reducing rotation of the rotating shaft and transmitting reduced rotation to the rotation output member; and a sliding bearing mounted between the rotation output member and the first fixed member so as to support the rotation output member in a rotatable manner on the first fixed member in the center axis direction.