PURPOSE: To stop the operation of a load and to prevent the load from being exposed to a danger even when a microcomputer breaks down by cutting off electric power supplied to the load by a switching means when the output of a constant voltage circuit rises above a specific voltage.

CONSTITUTION: This power unit is equipped with the constant voltage circuit which supplies a constant voltage to the microcomputer 6, a detecting means 5 which detects the output voltage of the constant voltage circuit to the microcomputer 6 rising above the specific voltage, and the switching SW which performs switching operation according to the detection state of the means 5. In this constitution, when the output of the constant voltage circuit to the microcomputer 6 exceeds the specific voltage, the detecting circuit 5 detects that and operates the switching means SW to cut off electric power supplied to the load controlled by this microcomputer 6. Consequently, even if the microcomputer 6 for the load DC5(V) breaks down owing to an abnormally high voltage, a heater H, a motor M, etc., controlled by the microcomputer 6 stop operating and never operate in an uncontrolled state.