PURPOSE: To make it possible to execute timely alteration of a phase difference angle for a phase of a power source by a simple control.

CONSTITUTION: A function of setting a phase addition angle command value for phase-shifting of a signal detected synchronously with a power source is added to a microcomputer 27, and phase addition angle data given from the microcomputer 27 are added in an adder 33 outside the microcomputer 27 to phase difference angle data being an output of an integrator circuit 31 and are outputted as final phase difference angle data. According to this constitution, a source sync signal shifted by an arbitrary phase from a phase of the power source can be obtained easily only by changing the phase addition angle data to be given to the adder 33, without altering each time a phase difference angle command value set inside the microcomputer 27 beforehand and making it execute a proportional-plus-integral operation.