To inexpensively cut off the transmission of torque from the output shaft side with a simple structure even if a separate brake mechanism is not used by providing a lock member for checking rotary operation of an output shaft.

A lock member 14 is arranged between an input shaft 12 and an output shaft 13. An inserting hole 14c capable of inserting a guide pin 13f projected on the output shaft 13 is arranged in an eccentric position dislocated from the center of this lock member 14. Since a projecting position of the guide pin 13f is the eccentric position of the output shaft 13, when the output shaft 13 rotates, the guide pin 13f becomes a rotary orbit such as moving around the axis of the output shaft 13. Thus, an inside surface of the inserting hole 14c is pressed, and since the inserting hole 14c also tries to become the same rotary orbit as the guide pin 13f, an outer peripheral surface of the lock member 14 tries to draw an outward projecting rotary orbit. Thus, since the outer peripheral surface of the lock member 14 is pressed to an inner peripheral surface of a casing 11, further rotation becomes impossible.