A system-interconnected inverter connected to a commercial system for supplying AC power, which inverter includes a switching device for turning on and off DC power based on a predetermined duty value to output a voltage whose frequency is the same as that of a system power source and a controller for controlling the switching device. The controller changes the duty value of the switching device in accordance with the value obtained by dividing the maximum value by the effective value of the output current from the switching device measured with the system-interconnected inverter disconnected from the system power source.