AC electric charge measurement device, and AC electric charge measurement method

According to a present alternating-current electric quantity measuring apparatus, a voltage amplitude calculated by a square integral operation of at least three continuous voltage instantaneous values sampled at a sampling frequency twice or higher than a frequency of an alternating-current voltage to be measured is normalized. A voltage chord length calculated by the square integral operation of three voltage chord length instantaneous values representing an end-to-end distance between two adjacent voltage instantaneous values in at least four continuous voltage instantaneous values including the three voltage instantaneous values, which are sampled at the sampling frequency and used in calculating the normalized voltage amplitude, is normalized. A rotation phase angle in one period time of sampling is calculated using the normalized voltage amplitude and the normalized voltage chord length. A frequency of the alternating-current voltage is calculated using the calculated rotation phase angle.