PURPOSE: To attain a wide variety of change in a time constant of an output voltage change with respect to an input power by providing an input/output comparison circuit detecting a voltage difference of input/output, a time constant setting device, an analog switch and an integration circuit.

CONSTITUTION: The input/output comparison circuit 1 consists of an inverse amplifier detecting a voltage difference between an input voltage and an output voltage and an analog switch drive circuit 12 switching analog switches 21, 23 in response to an output of the amplifier 11. Then the switches 21, 23 are switched via the circuit 12 in response to the level of the input/output voltage difference detected by the amplifier 11 and an integration input is given to an integration circuit 3 via a time constant setting device 24 from a DC power supply corresponding thereto at the closing of the switches. Thus, the feedback gain is not affected by the setting value of the setting device 24 and the time constant of the output voltage change with respect to the input voltage is converted over a wide variety.