A flue gas purifying device (16; 52; 72; 82) configured to reduce nitrogen oxides contained in flue gas discharged from a burning appliance, the device comprising: an exhaust pipe configured to guide flue gas discharged from the burning appliance; a urea-water injecting unit configured to inject urea water into the exhaust pipe; a catalytic unit that includes a urea SCR catalyst configured to promote a reaction between ammonia produced from injected urea water and the nitrogen oxides, and a support mechanism arranged inside of the exhaust pipe to support the urea SCR catalyst in the exhaust pipe, and that is arranged on a downstream side to a position where the urea water is injected in a flow direction of the flue gas; a first ammonia-concentration measuring unit configured to measure 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 (29, 76) arranged either 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, or alternatively in the catalytic unit on a downstream side to the first ammonia-concentration measuring unit, so as to be configured to measure an ammonia concentration in a region where the catalytic unit is arranged; and a control unit (30; 64; 78; 90) configured to control injection of urea water by the urea-water injecting unit based on measurement results acquired by the first ammonia-concentration measuring unit and the second ammonia-concentration measuring unit (29, 76),, characterized in that the control unit (30; 64; 78; 90) is configured to set a target concentration of the first ammonia-concentration measuring unit based on the measurement result acquired by the second ammonia-concentration measuring unit (29, 76), and to control injection of urea water by the urea-water injecting unit so that the measurement result acquired by the first ammonia-concentration measuring unit becomes the target concentration of the first ammonia-concentration measuring unit. an exhaust pipe configured to guide flue gas discharged from the burning appliance; a urea-water injecting unit configured to inject urea water into the exhaust pipe; a catalytic unit that includes a urea SCR catalyst configured to promote a reaction between ammonia produced from injected urea water and the nitrogen oxides, and a support mechanism arranged inside of the exhaust pipe to support the urea SCR catalyst in the exhaust pipe, and that is arranged on a downstream side to a position where the urea water is injected in a flow direction of the flue gas; a first ammonia-concentration measuring unit configured to measure 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 (29, 76) arranged either 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, or alternatively in the catalytic unit on a downstream side to the first ammonia-concentration measuring unit, so as to be configured to measure an ammonia concentration in a region where the catalytic unit is arranged; and a control unit (30; 64; 78; 90) configured to control injection of urea water by the urea-water injecting unit based on measurement results acquired by the first ammonia-concentration measuring unit and the second ammonia-concentration measuring unit (29, 76),, characterized in that the control unit (30; 64; 78; 90) is configured to set a target concentration of the first ammonia-concentration measuring unit based on the measurement result acquired by the second ammonia-concentration measuring unit (29, 76), and to control injection of urea water by the urea-water injecting unit so that the measurement result acquired by the first ammonia-concentration measuring unit becomes the target concentration of the first ammonia-concentration measuring unit.