PURPOSE: To obtain a program protection system which uses a high-speed power- failure detector, by detecting a power failure through a voltage comparator, a monostable device, etc., and then driving a high speed relay, and thus transferring a program in the initial setting state.

CONSTITUTION: The power from an AC power source is subjected to the full- wave rectification by a diode bridge circuit 1, whose output voltage is applied to and compared by a comparator 4 with a comparison voltage value set by a variable resistor 3, thereby controlling the output timing of an oscillator 5 on the basis of the comparison result of the comparator 4. Further, a power failure is detected by a monostable device 6 by using the output of the oscillator 5 to drive a high-speed relay 13 through a driver device 12, thereby initializing a program which is being executed by such as a microcomputer 7. Thus, the power failure is detected ≤10 millisecond after its occurrence and the program being executed is initialized to protect the program.