PURPOSE: To keep constant the sum of input and output signals with high speed response and stability, by applying the voltage corresponding to the sum of the currents which are in response to the input and output voltages, to one input terminal of the pulse width modulation circuit.

CONSTITUTION: The pulse width modulation circuit 6 compares the amplitude of the voltages fed to one input terminal and to another input, reference voltage, and produces the pulse width signal on the result. Further, the output signal is inputted to the primary coil of the conversion transformer 9 to obtain the AC voltage from the secondary coil. This AC output voltage is rectified 10 to obtain the DC output voltage, and the voltage co-responding to the sum currents in response to the input and output voltages is fed to one input terminal of the pulse width modulation circuit 6. Thus, the sum of input and output signals can be kept constant with high speed response and stability.