PURPOSE: To easily and with high accuracy find the peak value of a sine wave alternating current by averaging the peak value obtained by making double a difference of two adjacent data when the positive and negative codes of sampling data change and the peak value in which the largest data in the sam pling data is calculated as the maximum value of the alternating amount.

CONSTITUTION: Differential data is calculated by a differential data calculation part 2 based on the sampling data from a data extraction part 1, and the sampling data and the differential data are stored in a memory 6 in a time series way. A zero cross data extraction part 3 decides the difference between the positive and negative codes of two adjacent sampling data, and extracts the differential data which satisfied a specific condition from the memory 6, then, stores it in the memory 6 as zero cross data. Also, a largest data extraction part 4 extracts the sampling data in which two adjacent differential data satisfy the specific condition from the memory 6, and stores it in the memory 6 as the maximum value data. Next, the peak value is calculated by zero cross data and the maximum value data stored in the memory 6 at a peak value calculation part 5.