PURPOSE: To deal with unnecessary function instantaneously and positively by detecting differences between input voltage and current signals and output voltage and current signals and then making a judgment whether the differences are higher than predetermined values.

CONSTITUTION: Voltages from input and output main circuits in a semiconductor circuit breaker 12 are provided, for respective phases, to a differential voltage detector 141 in a protection/detection circuit 14 and then they are compared to one another. When a voltage difference passed through an amplifier 142 exceeds a predetermined value, a protection detecting output 144 is produced as an unnecessary function of the semiconductor circuit breaker 12 upon provision of on ON operation command 127 of the semiconductor circuit breaker 12. Since voltage drop in the main circuitry of the circuit breaker 12 is low, input and output voltages thereof are approximately equal to each other. Consequently, unnecessary function of the semiconductor circuit breaker 12 can instantaneously and reliably be detected based on the differential signals and a load can be protected.