A NOx reduction device suitable for reducing NOx from a lower temperature exhaust gas, said NOx reduction device being activatable at said lower temperature and having an exhaust gas main-passage from a main diesel engine of a ship diverted into a plurality of exhaust gas branch-passages each of which is provided with a NOx reduction catalyst and a first open-close valve for closing and opening the branch-passage through which the NOx reduction catalyst is controlled, thereby refreshment as to each of the NOx reduction catalysts is selectively performed in response to the temperature level of the exhaust gas, wherein the NOx reduction device further comprises: an auxiliary diesel engine, which is suitable for a power generation use and being provided in the ship besides the main diesel engine, and which is configured to emit a higher temperature exhaust gas than that of the main diesel engine, said higher temperature being set to exceed the temperature at which the NOx reduction catalyst can be refreshed, a plurality of communicating passages that respectively connect an exhaust passage of the auxiliary diesel engine and the plurality of the branch-passages that are diverted from the exhaust gas main-passage, wherein each of said communicating passages is provided with a second open-close valve; an exhaust gas bypass passage through which the exhaust gas from the auxiliary diesel engine bypasses the NOx reduction catalyst and joins an exhaust gas outlet and on which an exhaust gas bypass valve is provided; and a controller configured to control the opening and closing operation of the first and second open-close valves when at least one of the NOx reduction catalysts is to be refreshed, so that the higher temperature exhaust gas emitted from the auxiliary diesel engine goes through said at least one NOx reduction catalyst provided to the corresponding branch passage via the corresponding second open-close valve, thereby increasing the temperature of said at least one NOx reduction catalyst to a level enabling refreshment of said at least one NOx reduction catalyst. an auxiliary diesel engine, which is suitable for a power generation use and being provided in the ship besides the main diesel engine, and which is configured to emit a higher temperature exhaust gas than that of the main diesel engine, said higher temperature being set to exceed the temperature at which the NOx reduction catalyst can be refreshed, a plurality of communicating passages that respectively connect an exhaust passage of the auxiliary diesel engine and the plurality of the branch-passages that are diverted from the exhaust gas main-passage, wherein each of said communicating passages is provided with a second open-close valve; an exhaust gas bypass passage through which the exhaust gas from the auxiliary diesel engine bypasses the NOx reduction catalyst and joins an exhaust gas outlet and on which an exhaust gas bypass valve is provided; and a controller configured to control the opening and closing operation of the first and second open-close valves when at least one of the NOx reduction catalysts is to be refreshed, so that the higher temperature exhaust gas emitted from the auxiliary diesel engine goes through said at least one NOx reduction catalyst provided to the corresponding branch passage via the corresponding second open-close valve, thereby increasing the temperature of said at least one NOx reduction catalyst to a level enabling refreshment of said at least one NOx reduction catalyst.