PURPOSE: To improve accuracy in detecting synchronization of a phase of a basic wave and in detecting a phase and an amplitude of an alternating current voltage by switching a zero cross point phase detecting/operating unit and a Fourier converting/ operating unit of a PLL sampling data for a phase detecting unit of a digital PLL circuit in accordance with the deviation of the phase.

CONSTITUTION: When a PLL is withdrawn, a zero cross point of a signal digitized by an A/D14 is obtained by a zero cross point calculating/operating unit 22 on the basis of a sample instructing pulse of a sample timer 16. At this time, a switching means 34 is at the A side. Phase data of the zero cross point and inner phase data of a sample counter 18 are compared at a phase comparing/operating unit 26, and a sampling period is corrected in accordance with the deviation of the phase, with the PLL brought in the withdrawn state. After the PLL is completely withdrawn, the switching means is turned to the B side, whereby the phase data of a Fourier operating unit 24 is input to the other terminal of the operating unit 26. Accordingly, the phase of components of a base wave can be compared with high accuracy. The switching means 34 is switched by an output of a phase difference judging/operating unit 32.