To connect the phase controller, an AC power source, and a load by two wires, to perform replacement with a phase controller using a triac without altering wiring, and to actualize not only ordinary phase control, but also opposite-phase control operation by using a unidirectional MOSFET, etc., for an element which performs power control.

The electric power supplied to the load L is controlled by performing phase control by using two unidirectional switch elements Q1 and Q2 connected to a single-phase AC power source E reversely in series to perform ordinary phase control operation and opposite-phase control operation. This phase controller 1 has a control part 2 which uses one unidirectional switch element for the plus direction of an AC waveform and the other for the minus direction and controls the gate voltages of the unidirectional switch elements into a conduction mode and a nonconduction mode and a gate power source part G which supplies a gate voltage to the unidirectional switch elements, and the gate power source part G is formed of a power transformer T1 connected to an AC power source, thereby connecting the phase controller, power source, and load by two wires.