In the present invention, a mixture of fuel gas and air is combusted in a combustion chamber in a gas mode of a dual-fuel engine (1) for ships. The engine system is provided with: a control unit (22)for advancing the closing timing of an intake valve (8) of the engine and, so as to accord with the changes from said advancement, also advancing the opening timing of a fuel gas supply valve (15), when the output of an output shaft (2) of the engine (1) increases; a variable intake valve timing mechanism (30) for advancing the closing timing of the intake valve (8) in accordance with the intake valve (8) closing timing set by the control unit; and a fuel gas supply valve timing mechanism (45) for advancing the opening timing of the fuel gas supply valve (15) so as to accord with the changes from the advancement of the intake valve (8) set by the control unit. Control is performed such that with the increase in the output of the output shaft of the engine (1), the compression ratio of thegas-air mixture in the engine is further reduced by the variable intake valve timing mechanism (30).