PURPOSE: To obtain a stable output with high accuracy even when no load is connected by outputting the commanding value of a current amplifier circuit from the outputs of a voltage error circuit, a current error circuit and a deciding circuit deciding an output current to be zero.

CONSTITUTION: This control circuit is provided with a power amplifier circuit 2 connected in series to a DC power supply 1, a voltage detector 8 detecting the output voltage, a current detector 4 detecting the output current, a current reference circuit 7 obtaining a current reference value corresponding to the detected voltage, a voltage reference circuit 9 outputting a voltage reference at an open output and a control arithmetic circuit 6 outputting a command to the current amplifier circuit from the outputs of a current error circuit 5 and a voltage error circuit 10 and a deciding circuit 11 deciding whether or not an output current flows. Thus, the output of the current error circuit 5 at load connection and the output of the voltage error circuit 10 at no load are used respectively for an input to the control arithmetic circuit 6 and feedback control is constituted even when no load is connected.