PURPOSE: To enable prohibition of running of a compressor when a three-phase AC source is in a opposite phase by a method wherein the interline voltage, different from that for synchronizing a serial signal, of the interline voltages of the three-phase AC source is detected, a difference between the phase angle of the interline voltage and the phase angle of a serial signal is detected, and through comparison between the difference in a phase angle and a set value, it is decided whether the three-phase AC source is in a positive phase or in a counter phase.

CONSTITUTION: When a serial signal falls from an 'H' level to an 'L' level, a timer 42 is started. When an interline voltage input falls from an 'H' level to an 'L' level, the timer 2 is stopped, and an elapsing time (t) of the timer 42 is caused to correspond to a value which is higher than a set value α and lower than a set value β (α≤t≤β). Based on the above, it is decided that the three-phase AC source 1 is in a counter phase. It is confirmed whether or not the decision is the same as preceding decision, and when it is the same, '1' is added to a count valve X of an internal counter to repeat same control. When the count value X of the internal counter is in counter phase during decision made three times in succession, it is finally decided that the three-phase AC source 1 is in an opposite phase, and operation prohibiting processing is executed.