To accurately measure the root mean square value or mean value of AC voltage, regarding the measurement of the AC voltage.

A frequency detection means 4 outputs a frequency signal proportional to the frequency of detected AC voltage, and an integrating operation means 5 adds the absolute value of the digital signal matrix of AC voltage obtained with an A/D converter 2 by the predetermined times for outputting an integrated result. Furthermore, a phase difference angle operation means 6 finds a phase difference angle signal from the frequency signal of the AC voltage detected with the frequency detection means 4, expressing a digital signal conversion period in the form of the phase of the AC voltage. Also, a normalizing operation means 7 multiplies the integrated result by a voltage correction ratio obtained on the basis of the phase difference angle signal and, then, outputs a normalized voltage detection signal. As a result, an operation error is corrected.