An object of the present invention is to provide a flue gas purifying device that can efficiently decrease nitrogen oxides in flue gas. This object is solved by including: an exhaust pipe that guides flue gas discharged from a burning appliance; a urea-water injecting unit that injects urea water into the exhaust pipe; a catalytic unit arranged on a downstream side to a position where urea water is injected in a flow direction of flue gas and having a urea SCR catalyst; a first ammonia-concentration measuring unit that measures a concentration of ammonia in flue gas at a measurement position in a region where the catalytic unit is arranged; a second ammonia-concentration measuring unit arranged on a downstream side to the catalytic unit in a flow direction of the flue gas, to measure a concentration of ammonia in the flue gas having passed through the urea SCR catalyst; and a control unit that controls injection of urea water by the urea-water injecting unit based on measurement results acquired by the first and second ammonia-concentration measuring units.
