PURPOSE: To remove the force exerted to a channel selection member from a side and to surely prevent the production of malfunction surely, by orthogonal butt to a flat part of a tranpezoidal projection of a cam plate, when other channel selection members are depressed with an arbitrary one channel selection member depressed.

CONSTITUTION: A part of a device frame 1 is folded to form a front frame 2 and a rear frame 3, and a channel selection member 4 is fitted to the device frame 1, front frame 2 and rear frame 3. An operating piece 5 having push buttons 6 on the member 4 is located and a suppression piece 8 is given a slope so that the operating piece 5 can be slid on a metallic arm 7. A regulating plate 9 supported 11 freely turnably to the arm 7 is strongly suppressed with a lever operation. A folding piece 12 is located at the rear end of the arm 7 to constitute a lateral plate 13 as a whole, this is guided 14 freely transversally on the device frame 1, and when a regulating pin 10 is butted to a cam 15 of the plate 13, it is moved until the pin 10 is located on the top of the cam 15. A force exerted to the member 4 from the side when other members 4 are depressed with one member 4 depressed can be removed, allowing to avoid the production of malfunction.