PURPOSE: To detect a phase difference with high accuracy, by sampling two AC inputs from the same electric power source system, and using sampling values before and after zero-crossing of the AC inputs and a phase difference between each sampling.

CONSTITUTION: Two AC inputs inputted from the same electric power source system are sampled by a prescribed sampling frequency, and are converted to digital values. A negative sampling value X0 and a positive sampling value X1 in case when the input A1 is changed to positive from negative for to its polarity are derived, and in the same way, a negative sampling value X20 and a positive sampling value X21 of the input A2 are derived. Subsequently, a sampling number N0 extending from the sampling value X1 of the input A1 to the negative sampling value X20 of the input A2 is counted. Subsequently, a phase difference θ between each sampling in the actual power supply frequency is derived, phase difference α, β are derived by use of the sampling values X0, X1, X20 and X21 and the phase difference θ; the phase difference between two inputs is detected.