PURPOSE: To protect an amplifier from thermal destruction by decreasing a power supply voltage of the amplifier when the temperature of the amplifier rises.

CONSTITUTION: When an input signal applied to an input terminal 2 is large continuously, an output signal of the amplifier 1 is also large continuously, positive and negative high power supply voltages +B1, -B1 are applied to the amplifier 1, while an "H" signal is kept applied to a control input terminal of the 1st and 2nd power supply changeover circuits 4, 5. A large current flows to an output transistor of the amplifier 1, causing it to be heated. When the temperature rises and reaches a preset temperature, a detecting circuit 10 is actiated, a control signal is generated from the 1st and 2nd control signal generating circuits 11, 12 and it is applied to the 1st and 2nd power supply changeover circuits 4, 5 and an "L" signal respectively. Thus, the power supply changeover circuits 4 and 5 are changed over and positive, negative low power supply voltages +B2 and -B2 are applied to the amplifier 1.