To detect the supply interruption of negative voltage and at the same time, to stop the supply of positive voltage and to prevent the destruction of a power amplifier.

A power amplifier 1 uses the both voltage of positive voltage 10 and negative voltage 12 and performs a power amplification. A positive power source 5 outputs positive voltage 10. A negative power source 4 outputs negative voltage 12. A negative voltage detection circuit 2 detects the presence or absence of the negative voltage 12 and outputs a negative voltage detection signal 9. A switch control circuit 6 outputs a control signal 8. A switch circuit 3 controls the conduction interruption of the positive voltage 10 by the negative voltage detection signal 9 and the control signal 8.