PURPOSE: To prevent the unintentional stop of an apparatus in operation by locking output excepting the period of time immediately after the start of operation of said apparatus requiring the function of detecting a negative phase.

CONSTITUTION: When a load 2 is operated in the negative-phase state in (a) by a change of wiring, etc., a timer circuit 5 outputs 1 as in (c) at the beginning of electrification, a negative-phase detecting circuit 4 also outputs 1 as in (b), a transistor 7 is turned ON, an output relay 8 is excited as in (d), and said load 2 is separated from a power supply. Then, when said negative-phase detecting circuit 4 outputs 1 by mistake due to noise as in (e) at the time of the operating state of said load 2, said timer circuit 5 outputs 0 excepting a certain period at the be ginning of electrification so that the ON action of said transistor 7 is locked via diode 6. Therefore, the output relay 8 is neither excited nor outputted to the outside as in (f).