PURPOSE: To divide a potential of the primary power supply with resistors and detect a voltage drop to detect turning off of the power supply switch within a short period of time.

CONSTITUTION: A power switch 1 for supplying an AC 100V to a bridge rectifier circuit 2 and repeats the on-off operations, a smoothing capacitor 3, a power supply transformer 5 connecting its primary winding in parallel therewith to supply the predetermined voltage to a load circuit 15 through electrical insulation and a switch 12 for repeating the switching operations based on a switching control signal 9 to supply an AC to the primary winding are comprised. Moreover, a switching control circuit 8 for receiving the operation power source 11 from the primary side of the power supply to supply the switching control signal 9, a voltage detecting apparatus 6 for setting the smoothing capacitor 3 and a resistor 4 connected therewith in parallel to the predetermined voltage dividing ratio of R1, R2 to detect its divided voltage output and an optical transmission apparatus 7 for optically transmitting the detected signal and converts the received optical signal to an electrical signal 13 are also comprised.