PURPOSE: To realize the improvement of the safety and operability by a method wherein the fixing function of the movement of a locking lever is added onto an outer frame.

CONSTITUTION: A locking lever stopper 1 is pivotably mounted to the supporting part of the locking lever 2 of a gear cover 3 with a stepped screw 5. Further, a hole for engaging with the locking lever stopper 1 is bored at the rear end part of the locking lever 2. Under the condition that the locking lever stopper 1 mates with the projecting position of the locking lever 2, even if force is applied by mistake to the locking lever 2 during the movement of a main body, the locking lever 2 can not move, since the locking lever stopper 1 suppresses the projecting part of the locking lever 2, resulting in eliminating the danger for locking a rotor shaft.