PURPOSE: To enhance operational accuracy, by altering a sampling interval corresponding to frequency inputted.

CONSTITUTION: AC input is applied to an operational processing part 4 through a filter 1, a sample holding circuit 2 and an A/D converter circuit 3 but frequency operation other than amplitude value converting processing is performed in the processing part 4. A clock cycle is corrected on the basis of the frequency, which is obtained by said processing part 4, by a correction circuit 6 to correct sampling frequency so that data becomes 90° as an electric angle. The data becoming 90° as the electric angle is calculated by the sampling frequency corrected and, by this method, the enhancement of operational accuracy is achieved even when the frequency of an AC signal changes.