An exhaust gas purification device for a ship, comprising: a main path including a catalyst (34, 35) and a bypass path branching off from a midway of the main path, each of the main path and the bypass path serving as an exhaust gas path of an engine to be mounted in the ship; and a purification casing in which the main path and the bypass path are disposed, wherein parts of the main path and the bypass path are merged with each other for discharging an exhaust gas outside the purification casing, the parts being located downstream in an exhaust gas traveling direction, a partition plate by which the main path and the bypass path are partitioned from each other from an upstream side to a downstream side in the exhaust gas traveling direction is disposed in the purification casing and extends along the exhaust gas traveling direction, characterized in that the partition plate has a downstream end extending through between an exhaust gas outflow port of the purification casing and an exhaust gas outlet of the main path being extended toward the peripheral edge of the exhaust gas outflow port in the exhaust gas outlet of the purification casing without being bent and having an opening, the downstream end serving as a reverse-flow prevention plate, and the reverse-flow prevention plate is a perforated plate having the opening constituted by multiple holes, wherein the exhaust gas outflow port is in a location close to the bypass path, in a position where the exhaust gas outflow port overlaps the exhaust side of the bypass path, the downstream end of the partition plate is extended straightly along an exhaust gas traveling direction in the bypass path, and a downstream end of the reverse-flow prevention plate is in contact with a part of the inner wall surface of the downstream end of the purification casing, the part being closer to the main path side than the exhaust gas outflow port. a main path including a catalyst (34, 35) and a bypass path branching off from a midway of the main path, each of the main path and the bypass path serving as an exhaust gas path of an engine to be mounted in the ship; and a purification casing in which the main path and the bypass path are disposed, wherein parts of the main path and the bypass path are merged with each other for discharging an exhaust gas outside the purification casing, the parts being located downstream in an exhaust gas traveling direction, a partition plate by which the main path and the bypass path are partitioned from each other from an upstream side to a downstream side in the exhaust gas traveling direction is disposed in the purification casing and extends along the exhaust gas traveling direction, characterized in that the partition plate has a downstream end extending through between an exhaust gas outflow port of the purification casing and an exhaust gas outlet of the main path being extended toward the peripheral edge of the exhaust gas outflow port in the exhaust gas outlet of the purification casing without being bent and having an opening, the downstream end serving as a reverse-flow prevention plate, and the reverse-flow prevention plate is a perforated plate having the opening constituted by multiple holes, wherein the exhaust gas outflow port is in a location close to the bypass path, in a position where the exhaust gas outflow port overlaps the exhaust side of the bypass path, the downstream end of the partition plate is extended straightly along an exhaust gas traveling direction in the bypass path, and a downstream end of the reverse-flow prevention plate is in contact with a part of the inner wall surface of the downstream end of the purification casing, the part being closer to the main path side than the exhaust gas outflow port.