To reduce an integrated error by averaging received data and preceding received data for each arithmetic period.

An analog signal is sampled for each arithmetic period and the sampled signal is A/D-converted. Every time an arithmetic period timer is set, this value X1 of A/D-converted data is stored in a 1st memory and a preceding value X2 is stored in a 2nd memory (S2, S3), a mean value M of the X1, X2 is calculated (S4), and the obtained mean values M are integrated sequentially to obtain an integrated value S (S5). In the case of integrating the A/D-converted data without any arithmetic operation, let an arithmetic period be T, let a preceding value be A, and let this value be A+d, then the integrated preceding value is T.A, and the integrated this value is T.A+T.d, which is a stepwise value and an error is produced. However, the averaged value of the integrated this value M is T.A+T.d/2, resulting that the stepwise value is corrected by a slant component T.d/2. Thus, the integrated value S with a small error is obtained by integrating the mean values M sequentially.