PURPOSE: To improve the silence, smoothness and operability by turning on/off a hook switch in interlocking with a switching operation of a transmitter case and providing internally a lock mechanism to simplify the handling and to attain sure locking.

CONSTITUTION: A pressing section 15 is provided at a position corresponding to a switch button 14 and when the transmitter case 12 is folded onto a case 11, the hook switch 16 is turned off automatically and when the case 12 is opened, the hook switch 16 is turned on automatically respectively. Further, in switching the case 12 to the case 11, a locking projection 18a presses elastically an outer circumference 13c of the hinge shaft 13 to slide insert and to lock automatically to locking concaved parts 13a, 13b and the case 12 is fixed. Thus, the grambling sound is prevented from being produced because of a play of the hinge shaft 13, allowing to improve the silence, smoothness and operability at the switching of the case 12.