PURPOSE: To obtain method and apparatus for detecting the negative phase of three-phase four-wire system power supply for air-conditioner in which erroneous connection of power supply can be detected.

CONSTITUTION: A microcomputer 10 is fed, at the input ports P64, P65 thereof, with a voltage from a three-phase four-wire system power supply. When an edge detection circuit 1 detects the falling of input voltage to the P64, a counter set circuit 2 is set to be held in non-zero state for a phase interval of 90°. An edge detection circuit 3 detects the rising of input voltage to the P64 and when an edge is detected during the non-zero interval of counter, a negative phase detection timer set circuit 4 is set. The operation is repeated in the following period before the count of counter goes 0 thus recognizing a normal connection based on the output from the timer set circuit 4. In case of erroneous connection, the detection circuit 3 does not detect the falling during the non-zero interval of counter, and since the timer set circuit 4 is not set, the count of counter goes 0 thus recognizing the erroneous connection.