PURPOSE: To eliminate computing errors and enhance the computing accuracy by eliminating frequency fluctuations in AC input and the influence of sampling start phase.

CONSTITUTION: Basic term arithmetic means 10 determines the basic term by subtracting the product of the data at the reference sampling point in time and the data 180° in electrical angle ahead of the reference sampling point in time from a value obtained by squaring the data 90° in electrical angle ahead of the reference sampling point in time and by multiplying the result by the first coefficient. Correction term of the correction term arithmetic means 20 is determined by subtracting the product of data at the reference sampling point in time and the data 360° ahead of the reference sampling point in time from a value obtained by squaring the data 180° ahead of the reference sampling point in time and by multiplying the result by the second coefficient. For adding means 30, the basic term and correction term are added together, and the absolute value of AC input quantity is determined depending on the result.