PURPOSE: To precisely and easily discriminate ON/OFF of power supply with no time lag of detecting timing by taking in the variation of the power supply of prescribed time length in accordance with the processing time of the controlling means of an acoustic apparatus.

CONSTITUTION: When vehicle power supply to cause voltage drop instantaneously is inputted, a waveform shaping circuit 16 follows its change, but a time holding output signal C having been lowered by the action of a time holding circuit 17 holds the voltage of a low level for a definite period which can be selected in accordance with the processing time of a control part 19. A logical product is taken by a logical product circuit 18, and when the time holding output signal C of the time holding circuit 17 releases holding and returns to the voltage of a high level, when a digital voltage signal B from the waveform shaping circuit 16 is ON, the vehicle power supply is considered to be in a supplied state, and when the signal B is OFF at this time, it is decided by the control part 19 that the vehicle power supply is in a non-supplied state. Thus, the state of the vehicle power supply can be discriminated precisely and with no lag of timing without complicating the processing in the control part 19.