In a cogeneration system having a generator unit that generates DC power, an internal combustion engine for driving the generator unit, a battery that stores DC power for starting the engine, and an inverter that inverts the DC power generated by the generator unit to AC power, there are provided a solar unit that generates DC power by converting radiant energy of sunlight into electrical energy and a DC/DC converter that boosts the DC power stored in the battery, such that one of the battery and the solar unit is connected to the inverter through the DC/DC converter, thereby enabling them to share the DC/DC converter and thus to make the structure simple.