To prevent the deterioration of sound quality that is caused at a semiconductor switch part by deciding the sound volume according to the on/off actions of a semiconductor switch in a sound volume control mode and turning on a relay switch to bypass the semiconductor switch if the sound volume is not controlled for a prescribed time or longer.

The relay switches 61 to 63 are connected in parallel to the semiconductor switch of a gain switching circuit 53, e.g. between the collectors and emitters of transistors TR 54 to 56 and then turned on and off by a relay circuit 60 via the control signal that is sent from a controller 40. When the controller 40 decides that the position of a motor-driven volume 7 does not vary for a prescribed time, the switch 61 connected in parallel to the TR 54 that is currently turned on, for example, is turned off via the circuit 60. As a result, the TR 54 to which a signal is supplied is bypassed by the switch 61 and has no relation with a signal line.