A flue gas purifying device for reducing nitrogen oxides contained in flue gas discharged from an internal combustion engine, comprising: an exhaust pipe for guiding flue gas discharged from the internal combustion engine; a urea-water injecting unit for injecting urea water into the exhaust pipe; a catalytic unit that includes a urea SCR catalyst for promoting a reaction between ammonia produced from the injected urea water and the nitrogen oxides, and includes a support mechanism arranged inside of the exhaust pipe to support the urea SCR catalyst in the exhaust pipe, and is arranged on a downstream side to a position where the urea water is injected in a flow direction of the flue gas; an ammonia-concentration measuring unit arranged on a downstream side to the catalytic unit in a flow direction of the flue gas for measuring an ammonia concentration in the flue gas having passed through the urea SCR catalyst; a control unit for controlling injection of the urea water by the urea-water injecting unit based on the ammonia concentration measured by the ammonia-concentration measuring unit; a temperature adjusting unit for adjusting a temperature of a flue-gas flow path between the urea-water injecting unit and the catalytic unit in a flow direction of the flue gas; characterized in that the device comprises: at least one of an isocyanic-acid-concentration measuring unit for measuring a concentration of isocyanic acid in flue gas and a pretreatment ammonia-concentration measuring unit for measuring an ammonia concentration in flue gas, arranged between the urea-water injecting unit and the catalytic unit in a flow direction of the flue gas; and in that: the control unit is configured to cause the temperature adjusting unit to raise the temperature of the flue-gas flow path when a concentration of isocyanic acid measured by the isocyanic-acid-concentration measuring unit exceeds a predetermined value or when an ammonia concentration measured by the pretreatment ammonia-concentration measuring unit is below a predetermined value. an exhaust pipe for guiding flue gas discharged from the internal combustion engine; a urea-water injecting unit for injecting urea water into the exhaust pipe; a catalytic unit that includes a urea SCR catalyst for promoting a reaction between ammonia produced from the injected urea water and the nitrogen oxides, and includes a support mechanism arranged inside of the exhaust pipe to support the urea SCR catalyst in the exhaust pipe, and is arranged on a downstream side to a position where the urea water is injected in a flow direction of the flue gas; an ammonia-concentration measuring unit arranged on a downstream side to the catalytic unit in a flow direction of the flue gas for measuring an ammonia concentration in the flue gas having passed through the urea SCR catalyst; a control unit for controlling injection of the urea water by the urea-water injecting unit based on the ammonia concentration measured by the ammonia-concentration measuring unit; a temperature adjusting unit for adjusting a temperature of a flue-gas flow path between the urea-water injecting unit and the catalytic unit in a flow direction of the flue gas; characterized in that the device comprises: at least one of an isocyanic-acid-concentration measuring unit for measuring a concentration of isocyanic acid in flue gas and a pretreatment ammonia-concentration measuring unit for measuring an ammonia concentration in flue gas, arranged between the urea-water injecting unit and the catalytic unit in a flow direction of the flue gas; and in that: the control unit is configured to cause the temperature adjusting unit to raise the temperature of the flue-gas flow path when a concentration of isocyanic acid measured by the isocyanic-acid-concentration measuring unit exceeds a predetermined value or when an ammonia concentration measured by the pretreatment ammonia-concentration measuring unit is below a predetermined value. at least one of an isocyanic-acid-concentration measuring unit for measuring a concentration of isocyanic acid in flue gas and a pretreatment ammonia-concentration measuring unit for measuring an ammonia concentration in flue gas, arranged between the urea-water injecting unit and the catalytic unit in a flow direction of the flue gas; and in that: the control unit is configured to cause the temperature adjusting unit to raise the temperature of the flue-gas flow path when a concentration of isocyanic acid measured by the isocyanic-acid-concentration measuring unit exceeds a predetermined value or when an ammonia concentration measured by the pretreatment ammonia-concentration measuring unit is below a predetermined value.