The present invention makes it possible for total operating costs to be reduced according to the demand for power in a ship without a diesel engine (auxiliary engine) which has higher fuel consumption than a marine diesel engine (main engine) being excessively operated. If the opening degree of a gas inlet control valve (V1) reaches a predetermined threshold value and the amount of electricity generated by a generator (11) falls below the amount of power necessary in the ship, in accordance with the difference between the amount of electricity generated by the generator (11) and the amount of power necessary in the ship, a control command is output from a control unit (23) to an engine main body (2) so as to delay the timing at which fuel is injected and/or speed up the timing at which an exhaust valve is opened.