PURPOSE: To improve the reliability of an inverse connection preventing switching circuit by realizing the circuit by controlling the drain electrodes of a P-and N-type MOS transistors with a voltage obtained by dividing a voltage extracted from two photocouplers.

CONSTITUTION: Even when a switch 26 is turned on by connecting a positive-pole power supply input terminal 21 to the minus side and a negative-pole power supply input terminal 22 to the plus side, transistors are left opened, since the diodes of photocouplers 11 and 12 are biased in the opposite direction. However, since the diodes 24, 25, 27, and 28 for preventing inverse voltage of the photocouplers 11 and 12 are conducted and the gates of an N-and P-type MOS transistors 34 and 33 are biased respectively to negative and positive potential against their sources, the transistors 33 and 34 are not conductive between their drains and sources and no electric power is supplied to load terminals 35 and 36.