A cogeneration system includes a fuel evaporator; an internal combustion engine which outputs mechanical energy by combusting fuel evaporated by the fuel evaporator; an electric energy generator that converts the mechanical energy to an electric energy; a cold energy recovery portion which recovers cold energy generated when the fuel evaporator evaporates the fuel; an exhaust heat recovery portion which recovers heat exhausted from the engine; and a gas cleaner that cleans nitrogen oxides contained in gas exhausted from the engine. When the fuel is supplied to the gas cleaner, the fuel reduces the nitrogen oxides by reacting with the nitrogen oxides.