First and second turbochargers for supercharging a first cylinder group which continues or stops an operation by a valve stopping mechanism for reduced cylinders, and a second cylinder group which stops or continues the operation by the valve stopping mechanism for reduced cylinders, respectively, and a third turbocharger in which air from the first and second turbochargers is supplied to the first and second cylinder groups via separate independent intake passages, to supercharge the first and second turbochargers. If a reduced-cylinder operation system provided with this valve stopping mechanism is provided in addition to the turbo-charging system, a drop in a supercharging amount during a reduced-cylinder operation is prevented, a sufficient air amount is supplied into operating cylinders, deterioration of combustion in the operating cylinders and deterioration of a state of an exhaust gas are prevented, and fuel efficiency is improved.