An exhaust gas purification device for a ship including: a main path communicated with outside and a bypass path branching off from a midway portion of the main path, each of the paths serving as an exhaust gas path of an engine to be mounted in the ship. A partition plate, by which the main and bypass paths are partitioned from the upstream side to the downstream side in an exhaust gas traveling direction, extends along the exhaust gas traveling direction. The partition plate has a downstream end extended through between an exhaust gas outflow port of the purification casing and an exhaust gas outlet of the main path, and the downstream end is formed with an opening to serve as a reverse-flow prevention plate.