To surely prevent an operation mistake by forming a lock mechanism between an operation knob and a body chassis and simultaneously fixing the movement in the direction of turning with the knob thrust.

Relating to an operation knob 10 which is fixed at a position that is different from a normal state when it is thrust and returns to the position in the normal state when the knob 10 is thrust once more, a lock mechanism is formed between the rear plane of the knob 10 and a body chassis 30, the knob 10 is fixed, and also, the movement in the direction of turning is fixed when the knob 10 is thrust. The lock mechanism has at least one projection 4p on the rear plane of the knob 10, and holes 42 for locking into which the projection 40 is interfitted are provided on the chassis 30.