PURPOSE: To make it possible to obtain a synchronous phase signal by quickly following up to a sudden change in the voltage phase of an AC power supply by controlling the slope of a second triangular wave signal in such a manner that the phase difference between first and second triangular wave signals forming a particular phase respectively will become zero.

CONSTITUTION: A triangular wave signal converting circuit 1 outputs two triangular wave signals, one of which has the same phase as that of an AC input signal and the other has a 90-degree phase difference. Signal generators 8 and 9 output two triangular wave signals having the same positive and negative amplitude difference and having 90-degree phase difference. Multipliers 3 and 4 and a subtractor 5 determines a phase difference between both the triangular wave signals output from the triangular wave signal converting circuit 1 and signal generators 8 and 9 respectively. A slope operator 7 inputs said phase difference decided and determines a slope between these two triangular wave signals output by the signal generators 8 and 9. By doing this, it becomes possible to obtain synchronous phase signals by quickly following up a sudden change in the voltage phase of an AC power supply.