In a unit inverter system where multiple unit inverters are connected in parallel, the quantity of operating unit inverters is determined in accordance with an amount of power to be converted. A gate signal of a semiconductor switching element of a unit inverter is turned off after an output current of the inverter is reduced when reducing the quantity of inverter units, thereby improving the partial load efficiency of the system without an adverse effect on the system. A regulator connected to the inverter determines dead time of the inverter according to the output current value and an average output current value of the unit inverters, waits for the determined dead time so as to reduce the output current of the unit inverter, and then turns off the gate signal.