PURPOSE: To set turning-on timing of a synchronous interrupter properly, by judging a condition where only phase difference exists when a synchronous machine is started synchronously and providing a speed incremental/decremental command signal corresponding to the phase difference to a frequency regulation loop.

CONSTITUTION: Means 10 for detecting voltage difference, means 12 for detecting frequency difference and means 14 for detecting phase difference are provided and detected values are fed to a phase control judging means 16. The judging means 16 judges a condition where there is no voltage difference nor frequency difference but only phase difference exists, and provides a signal to a phase control means 18. The phase control means 18 provides a signal to a frequency regulator 20 forming a frequency regulation loop for a synchronous machine corresponding to the signal fed to the phase control means 18 and temporarily locks modification of setting level of a frequency setter. Then a command for incrementing or decrementing speed for a predetermined time is provided to a frequency regulation loop so as to eliminate phase difference.