A speed reducer 32 has an eccentric rotating member 56 which rotates along with a motor shaft 55 of an electric motor 11, an outer gear 57 which is rotatably supported by the outer circumference of the eccentric rotating member 56, an inner gear 58 which is fixedly disposed so as to be coaxial with the motor shaft 55 so as to engage with the outer gear 57, an output member 59 which is connected to an input member 37 of the motion converting mechanism 22 disposed so as to be coaxial with the motor shaft 55 so as to be not relatively rotatable about the input member 37, and an Oldham mechanism 60 which is disposed between the outer gear 57 and the output member 59 so as to transmit only a rotational component of the outer gear 57.