A combustion system for ships operated at low cost is provided. A combustion system 1 for ships includes an internal combustion engine 20 that burns fuel, an exhaust line L2 through which exhaust gas flows, the exhaust gas being generated through combustion of the fuel in the internal combustion engine 20, an exhaust heat recovery device 40 that is disposed in the exhaust line L2 and that recovers exhaust heat from the exhaust gas discharged from the internal combustion engine 20, and a denitration device 50 that is disposed in the exhaust line L2 and that removes nitrogen oxide from the exhaust gas using a denitration catalyst. The denitration device 50 is disposed downstream from the exhaust heat recovery device 40 in the exhaust line L2. The denitration catalyst contains 43 wt % or more of vanadium pentoxide and has a BET specific surface area of 30 m2/g or more.