PURPOSE: To stop the operation of a power source circuit by actuating all overvoltage protective circuits when either one of detected overvoltages is inputted by inputting signals from detecting sections which detect overvoltages from each DC output voltage and connecting the overvoltage protective circuits to an output terminal so that the circuits can be actuated simultaneously.

CONSTITUTION: When all power source blocks operate normally, no voltage is generated across a common resistor R1 and, as a result, a transistor TR2 is turned on, because no electric current flows to Zener diodes D1 and D2. In the case a DC output voltage Va1 abnormally rises for some reason and the voltage applied across the diode D1 becomes higher than a Zener diode Vz, the diode D1 is conducted and a voltage is generated across the resistor R1, resulting in the turning on of the transistor TR2. Consequently, transistors TR3 and TR4 are turned on and power source blocks 2 and 3 operate. In addition, the switching control by a controller 2b is stopped and power supply generating operations are simultaneously stopped.