An exhaust gas purifying system of an engine in a marine vessel comprising: an engine; the engine being to be loaded in the marine vessel, an NOx included in an exhaust gas of the engine being purified by spraying a reducing agent water solution to a reducing catalyst provided in an exhaust pipe of the engine; and wherein the exhaust gas purifying system has a raw material hopper in which the raw material reducing agent in solid-state, such as a powder state or a granular state, is thrown, the exhaust gas purifying system having a mixing tank with agitating means (38, 41)for mixing the raw material reducing agent discharged from the raw material hopper with water so as to form a water solution, characterized by a water generating machine for changing a sea water to a real water; a water tank for reserving the real water produced by the water generating machine; the water generating machine, and the water tank being to be loaded in the marine vessel; the reducing agent water solution being manufactured by mixing a raw material reducing agent in solid-state with the real water, the water tank and the mixing tank are connected by a raw water conduit line, and a water solution conduit line (29, 31) leading the reducing agent water solution to the reducing catalyst is connected to the mixing tank, the exhaust gas purifying system is provided with an output sensor for detecting an output of the engine or a concentration sensor for detecting an NOx concentration in the exhaust gas, a regulation tank for regulating a concentration of the reducing agent by diluting the reducing agent water solution supplied from the mixing tank by the real water is interposed in the water solution conduit line (29, 31), the concentration of the reducing agent in the regulation tank is regulated on the basis of a signal from the output sensor or the concentration sensor, and a water is supplied to said regulation tank from the water tank. an engine; the engine being to be loaded in the marine vessel, an NOx included in an exhaust gas of the engine being purified by spraying a reducing agent water solution to a reducing catalyst provided in an exhaust pipe of the engine; and wherein the exhaust gas purifying system has a raw material hopper in which the raw material reducing agent in solid-state, such as a powder state or a granular state, is thrown, the exhaust gas purifying system having a mixing tank with agitating means (38, 41)for mixing the raw material reducing agent discharged from the raw material hopper with water so as to form a water solution, characterized by a water generating machine for changing a sea water to a real water; a water tank for reserving the real water produced by the water generating machine; the water generating machine, and the water tank being to be loaded in the marine vessel; the reducing agent water solution being manufactured by mixing a raw material reducing agent in solid-state with the real water, the water tank and the mixing tank are connected by a raw water conduit line, and a water solution conduit line (29, 31) leading the reducing agent water solution to the reducing catalyst is connected to the mixing tank, the exhaust gas purifying system is provided with an output sensor for detecting an output of the engine or a concentration sensor for detecting an NOx concentration in the exhaust gas, a regulation tank for regulating a concentration of the reducing agent by diluting the reducing agent water solution supplied from the mixing tank by the real water is interposed in the water solution conduit line (29, 31), the concentration of the reducing agent in the regulation tank is regulated on the basis of a signal from the output sensor or the concentration sensor, and a water is supplied to said regulation tank from the water tank.