To operate a power supply protection circuit by comparing in direct two voltage differences of the power supply circuit, rather than determining whether the outputs of two systems of power supply circuit are normal or irregular by respectively comparing these outputs with the reference voltage.

This power supply protection circuit switches off the power supply operation through a process that a first output of the power supply circuit 5 is divided with a resistor and is then inputted to a positive side of an operational amplifier 9, a second output of the power supply circuit 5 is also divided with resistor and is then inputted to a negative side of the operational amplifier 9, each resistor division is set so that the positive side input of the operational amplifier 9 becomes lower than the negative side input, an output of the operational amplifier 9 is 0 V in normal condition, but an output of the operational amplifier 9 is inverted when a difference of two outputs becomes higher than the constant level. When such a difference is increased up to the power supply voltage of the operational amplifier 9, this signal is inputted to a microcomputer.