PURPOSE: To enable reliability to be improved and cost to be reduced with a simple circuit configuration by dividing a phase difference signal which is in proportion to a multi-phase AC voltage by the amplitude and reducing influ ence of temperature drift.

CONSTITUTION: After a three-phase AC voltage is rectified by an all-wave rectification circuit 12, it is converted into digital data by an AD converter 13 and a CPU 14 which controls an entire device reads a voltage amplitude V. Then, an inverse number of the voltage amplitude V is calculated by software utilizing spare time of the CPU 14 and is output to a multiplication-type DA converter 4E, where VsinΔθ which is obtained from a three-phase AC voltage through a three-phase and 2-phase conversion circuit 1 is multiplied by the inverse number of its amplitude V, thus enabling sinΔθ to be output. The detection phase θ' is obtained through a low-pass filter 8 and then a counter 11, thus enabling influence of change in voltage amplitude to be eliminated with a simpler circuit.