PROBLEM TO BE SOLVED: To provide a maritime exhaust gas denitration device capable of reducing running cost, by restraining fuel consumption in a heating means.SOLUTION: This maritime exhaust gas denitration device includes a reducing agent storage tank 11 for storing a reducing agent, a reducing agent injection nozzle 13 for jetting the reducing agent stored in the reducing agent storage tank 11 into exhaust gas, a denitration reactor 14 having a catalyst for reducing nitrogen oxides included in the exhaust gas, a heating means 15 arranged on the upstream side of the denitration reactor 14 and raising the temperature of the exhaust gas supplied to the denitration reactor 14, and a control device 16 having the function of operating the heating means 15 when performance of the denitration reactor 14 becomes less than a predetermined level to raise the temperature of the exhaust gas supplied to the denitration reactor 14, and afterwards stopping the heating means 15 and stopping a temperature rise in the exhaust gas supplied to the denitration reactor 14, when the performance of the denitration reactor 14 exceeds the predetermined level.