An exhaust gas denitration system, comprising: an exhaust gas turbocharger (5; 5'; 5") driven by exhaust gas from a marine diesel engine; and a denitration catalyst part which is provided in an exhaust gas path on a downstream side of the exhaust gas turbocharger (5; 5'; 5") and which denitrates the exhaust gas characterized in that the system further comprises: a reactor which produces ammonia from urea water; a urea water injection nozzle which supplies the urea water into the reactor, wherein the denitration catalyst part denitrates the exhaust gas by using the ammonia supplied from the reactor; an exhaust gas bypass path through which a part of the exhaust gas, which is obtained by partly extracting the exhaust gas being supplied to the exhaust gas turbocharger (5; 5'; 5") from the marine diesel engine, flows into the reactor; a bypass control valve which adjusts a flow rate of the exhaust gas flowing through the exhaust gas bypass path, and a scavenging pressure controller which controls scavenging pressure of the marine diesel engine in accordance with an amount of the exhaust gas flowing through the exhaust gas bypass path, wherein, when the amount of the exhaust gas flowing into the exhaust gas bypass path is reduced, the scavenging pressure controller reduces the scavenging pressure, and, when the amount of the exhaust gas flowing into the exhaust gas bypass path is increased, the scavenging pressure controller increases the scavenging pressure. an exhaust gas turbocharger (5; 5'; 5") driven by exhaust gas from a marine diesel engine; and a denitration catalyst part which is provided in an exhaust gas path on a downstream side of the exhaust gas turbocharger (5; 5'; 5") and which denitrates the exhaust gas characterized in that the system further comprises: a reactor which produces ammonia from urea water; a urea water injection nozzle which supplies the urea water into the reactor, wherein the denitration catalyst part denitrates the exhaust gas by using the ammonia supplied from the reactor; an exhaust gas bypass path through which a part of the exhaust gas, which is obtained by partly extracting the exhaust gas being supplied to the exhaust gas turbocharger (5; 5'; 5") from the marine diesel engine, flows into the reactor; a bypass control valve which adjusts a flow rate of the exhaust gas flowing through the exhaust gas bypass path, and a scavenging pressure controller which controls scavenging pressure of the marine diesel engine in accordance with an amount of the exhaust gas flowing through the exhaust gas bypass path, a reactor which produces ammonia from urea water; a urea water injection nozzle which supplies the urea water into the reactor, wherein the denitration catalyst part denitrates the exhaust gas by using the ammonia supplied from the reactor; an exhaust gas bypass path through which a part of the exhaust gas, which is obtained by partly extracting the exhaust gas being supplied to the exhaust gas turbocharger (5; 5'; 5") from the marine diesel engine, flows into the reactor; a bypass control valve which adjusts a flow rate of the exhaust gas flowing through the exhaust gas bypass path, and a scavenging pressure controller which controls scavenging pressure of the marine diesel engine in accordance with an amount of the exhaust gas flowing through the exhaust gas bypass path, wherein, when the amount of the exhaust gas flowing into the exhaust gas bypass path is reduced, the scavenging pressure controller reduces the scavenging pressure, and, when the amount of the exhaust gas flowing into the exhaust gas bypass path is increased, the scavenging pressure controller increases the scavenging pressure.