To effectively eliminate the generation of discharge phenomenon, damage, and noises by detecting the voltage zero point of an AC power supply and turning ON a contact at the current zero point of a coil based on this detection.

A microcomputer 8 detects the timing of the rising and falling of rectangular waves output from a power detection circuit 9, precisely grasps a high potential time, and detects the voltage zero point of the AC power supply based on this detection. And, the microcomputer 8 calculates the value of the impedance of a coil 6 of a relay 2 based on the voltage zero point of AC power supply and obtains the current zero point of the coil 6. When power supply to a load 4 starts, a contact 7 is turned ON at the current zero point of the coil 6. Thereby, discharge phenomenon generated in the relay contact and the coil electric circuit is inhibited, equipment damage and noise generation are effectively eliminated. Further, there is no need for providing an expensive switching element for controlling power supply to the coil, and cost reduction can be ensured. If power supply to the load is turned OFF, the contact point is turned OFF in the coil current zero point.