An exhaust gas purification system of the present invention includes, as an exhaust gas path 30 of an engine 25 to be mounted in a ship 1, a main path 31 which is in communication with outside, a bypass path 32 which branches off from a halfway portion of the main path 31, and a combined casing 33 with which both the main path 31 and the bypass path 32 are in communication. A selective catalyst reduction device 34, 35 is accommodated in the combined casing 33 at a location close to the main path 31. A path-switching member 37, 38 which switches exhaust gas moving direction is placed in a branched portion 53 between the main path 31 and the bypass path 32. A reducing agent injection body 61 is placed in the main path 31 between the path-switching member 37 and the combined casing 33.