PURPOSE: To inform an operator of the inappropriateness of supply voltage without damaging an electric circuit by outputting voltage driving a switching element when input voltage exceeds a fixed value, interrupting DC power fed to the load side when said switch is turned ON and informing the operator of the abnormality of input voltage by light and a sound.

CONSTITUTION: When input voltage satisfies specifications, a photo-coupler 7 is not driven, and a section between terminals (c)-(d) is held under an OFF state, thus supplying the load side with DC power. When input voltage takes a value exceeding specifications, currents are made to flow through a coil L because the photo-coupler 7 is driven instantaneously and the section between the terminals (c)-(d) is brought to an ON state, a relay switch 8 is changed over from a terminal S1 to a terminal S2, DC power fed to the load side is interrupted and applied to a light-emitting element 9 and a buzzer 10, and an operator is warned of the abnormality of supply voltage.