PURPOSE: To detect phase difference with high speed without the use of multipying and dividing methods by the procedure in which the effect of Dc current off-set content is reduced by newly coding the increase and decrease of sampling value on the basis of differential operation and further by detecting the phase difference in the coded value only by addition and substraction method.

CONSTITUTION: At the steps 31 and 32, the time of sampling at which the signal of the rows JA(m) and JB(m) of the AC amount A and B is inversed is detected, and then by using the values JA(m), RA(m), JB(m), and KB(m) at this time, the sampling phase difference is detected at the steps 33 and 34. On the basis of the sampling phase difference condition, the semicycle addition values SA(m) and SB(m) are determined at the stops 35 to 37, and then the max. absolute value of the differences is detected at the step 38. And, using judged value at the estep 33, the signal inversion time of the rows AJ(m) and JB(m) of the Ac current amount A and B is obtained at the step 43.