To prevent a DC overvoltage which is generated when active power flows in during the transitional period of the interconnection of an inverter to a power system by setting a mode in which active power is outputted from the inverter during the transitional period by advancing the phase of the output voltage of the inverter when the inverter is interconnected to a power system by a specific amount against a system voltage.

A multiplier 12 generates an control signal VE which controls the active power of an inverter by multiplying the sum signal gE(gE or gE+gC) of the output signal gE of a DC voltage control circuit 11 and a phase bias signal GC by a phase signal VS obtained by delaying the phase of a system voltage VS by 90° by means of a phase shifter 13. Since the phase bias signal g. is added. the inverter always operates from a state where the inverter is advanced from the output voltage VS of the inverter at the interconnecting time when the inverter is interconnected to a power system. Therefore, the generation of a DC overvoltage from the inverter is prevented during the transitional period of the interconnection, because the inverter is set to a mode in which the inverter outputs active power and suppresses the flowing in of active power.