To provide a phase difference control circuit for adjusting the phase difference between the voltage waveform and current waveform of an AC output from a power source to a load into an optimal state.

A phase difference control circuit 1 is provided with a control part 1a, arranged at the primary coil side of a transformer T in a power source 3 which is being the primary part of a power source device 2, a phase difference detecting part 1b arranged at a secondary coil side, and a photo-coupler PI1 linking the control part 1a with the phase difference detecting part 1b. In this case, the capacitor of the control part 1a is charged by the phase difference detecting part 1b according to the magnitude of the phase difference between the voltage waveform and current waveform of an AC output given to a load (not shown by a figure) by the power source device 2. Thus, the charging time can be made longer when the phase difference is larger, and a control voltage is also made larger. The voltage of the capacitor C is applied to a power source part 3, so that the power source part 3 can adjust the phase difference according to the voltage to be applied.