DESCRIPTION FLUE GAS PURIFYING DEVICE Field [0001] The present invention relates to a flue gas purifying device that reduces nitrogen oxides discharged from a burning appliance. Background [0002] Gas discharged from burning appliances such as an internal combustion engine, a waste incinerator, and a gas turbine, that is flue gas, contains nitrogen oxides (NOx). Therefore, a device that decreases nitrogen oxides is provided in an exhaust pipe of a burning appliance. As an example of the device that decreases nitrogen oxides, there is a device that decreases nitrogen oxides from flue gas by injecting urea into an exhaust pipe that guides flue gas, produces ammonia from urea in the exhaust pipe, causes the produced ammonia to react with nitrogen oxides in flue gas, and then removes oxygen from nitrogen oxides to produce nitrogen again. [0003] For example, Patent Literature 1 describes a flue gas purifying device in which a DPF device and a selective catalytic reduction catalytic device are sequentially arranged from an upstream side in an exhaust path of an internal combustion engine. Patent Literature 1 also describes a device that calculates NOx emissions, at the time of a normal operation, based on an NOx emissions map for the normal operation, or at the time of forced regeneration of the DPF device, calculates NOx emissions based on an NOx emissions map for forced regeneration, to calculate a feed rate of ammonia aqueous solution corresponding to the calculated NOx emissions, and feeds ammonia aqueous solution into flue gas on an upstream side of the selective catalytic reduction catalytic device so as to reach the calculated feed rate. [0004] Further, Patent Literature 2 describes NOx removal equipment for flue gas discharged from a combustion plant such as a waste incinerator. Patent Literature 2 describes a denitration control method in which a NOx concentration in gas before treatment, an ammonia concentration in treated flue gas, a NOx concentration in flue gas, and a flow rate of flue gas are measured, to calculate a flow rate of NOx before treatment, a NOx concentration after treatment, a record of NOx removal efficiency by NOx removal equipment, and an ammonia concentration in treated flue gas based on a measurement result thereof, deviations between the calculated values and target concentrations thereof are respectively calculated to thereby calculate correction values based on the calculated deviations, and a corrected flow rate of NOx is calculated based on at least one of the calculated correction values, thereby controlling a flow rate of ammonia to be injected into flue gas before treatment based on the calculated corrected value of NOx. Citation List Patent Literature [0005] Patent Literature 1: Japanese Patent Application Laid-open No. 2007-154849 Patent Literature 2: Japanese Patent Application Laid-open No. 2005-169331 Summary Technical Problem [0006] As described in Patent Literature 1, nitrogen oxides can be decreased by controlling an injection amount of urea based on a map created beforehand, and an amount of ammonia can be also adjusted. As described in Patent Literature 2, nitrogen oxides can be decreased as well by-using at least one of the concentrations of nitrogen oxides, NOx removal efficiency, and an ammonia concentration in treated flue gas to correct a deviation of flow rate of nitrogen oxides, and the amount of ammonia can be also adjusted. [0007] However, even if the injection amount of urea is adjusted based on the map created beforehand as described in Patent Literature 1, there are problems such as leakage of nitrogen oxides and leakage of ammonia according to operating conditions. To calculate the flow rate of NOx as described in Patent Literature 2, a calculation needs to be performed by detecting a flow rate of flue gas and the concentration of NOx (nitrogen oxides), thereby causing a problem that the calculation takes time. Further, because emissions of the internal combustion engine change greatly, there is a problem that the flow rate of NOx is difficult to calculate. There is another problem that even if the injection amount of ammonia is controlled based on the flow rate of NOx, the amount of nitrogen oxides and leakage of ammonia cannot be sufficiently decreased. [0008] Further, even when the injection amount of urea is adjusted based on the concentration of nitrogen oxides or an ammonia concentration in treated flue gas, if ammonia remains in the treated flue gas, which is a detection target, ammonia may leak. Therefore, in an air pollution control device of the internal combustion engine, an oxidation catalyst for oxidizing ammonia is installed on a downstream side of NOx removal equipment such as a selective-catalytic-reduction catalytic device. However, there is a problem that nitrogen oxides are produced due to oxidization of ammonia. Further, if the leaked amount of ammonia is large, the size of the oxidation catalyst needs to be increased. [0009] The present invention has been achieved to solve the above problems, and an object of the present invention is to provide a flue gas purifying device that calculates an appropriate amount of urea to be injected into an exhaust pipe so that ammonia hardly leaks to a downstream side, thereby efficiently decreasing nitrogen oxides in flue gas. Solution to Problem [0010] According to a aspect of the present invention, a flue gas purifying device that reduces nitrogen oxides contained in flue gas discharged from a burning appliance includes: an exhaust pipe that guides flue gas discharged from the burning appliance; a urea-water injecting unit that injects urea water into the exhaust pipe; a catalytic unit that includes a urea SCR catalyst that promotes 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 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. [0011] As described above, by controlling injection of urea water performed by the urea-water injecting unit by the control unit based on the concentration of ammonia contained in flue gas in the middle of the urea SCR catalyst and ammonia contained in flue gas having passed through the urea SCR catalyst, nitrogen oxides in flue gas can be decreased, while further decreasing ammonia in flue gas discharged from the flue gas purifying device. [0012] Advantageously, in the flue gas purifying device, the control unit sets a target concentration of the first ammonia-concentration measuring unit based on the measurement result acquired by the second ammonia-concentration measuring unit, and controls 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. In this manner, by setting the target value of the ammonia concentration on an upstream side based on the ammonia concentration on a downstream side, nitrogen oxides in flue gas can be decreased, while further decreasing ammonia in flue gas discharged from the flue gas purifying device. [0013] Advantageously, in the flue gas purifying device, the control unit controls injection of urea water by the urea-water injecting unit, such that, when it is assumed that a calculation basis of the target concentration at a measurement position of the first ammonia-concentration measuring unit is C1(NH3), the target concentration at the measurement position of the first ammonia-concentration measuring unit is C1(NH3)', the measurement result acquired by the second ammonia-concentration measuring unit is C2(t, NH3), and the target concentration at a measurement position of the second ammonia-concentration measuring unit is C20(NH3), C1 NH3)' is C1 (NH3)'=C1 (NH3)/(C2(t, NH3) /C20 (NH3) +0 . 5) and the measurement result acquired by the first ammonia-concentration measuring unit becomes C1(NH3)' . [0014] By controlling injection of urea water using the above equation, nitrogen oxides in flue gas can be decreased, while further decreasing ammonia in flue gas discharged from the flue gas purifying device. [0015] Advantageously, in the flue gas purifying device, the control unit controls injection of urea water by the urea-water injecting unit, such that, when it is assumed that a calculation basis of the target concentration at a measurement position of the first ammonia-concentration measuring unit is C1NH3), the target concentration at the measurement position of the first ammonia-concentration measuring unit is C1(NH3)', the measurement result acquired by the second ammonia-concentration measuring unit is C2(t, NH3), the target concentration at a measurement position of the second ammonia-concentration measuring unit is C20(NH3), and an arbitrary constant is n, C1(NH3)' is C1(NH3) ' =C1 (NH3) -nx(C2(t, NH3)-C20 (NH3) x0. 5) and the measurement result acquired by the first ammonia-concentration measuring unit becomes C1 (NH3) ' . [0016] By controlling injection of urea water using the above equation, nitrogen oxides in flue gas can be decreased, while further decreasing ammonia in flue gas discharged from the flue gas purifying device. [0017] Advantageously, in the flue gas purifying device, the control unit controls injection of urea water by the urea-water injecting unit, such that, when it is assumed that a calculation basis of the target concentration at a measurement position of the first ammonia-concentration measuring unit is C1(NH3), the target concentration at the measurement position of the first ammonia-concentration measuring unit is C1NH3)', the measurement result acquired by the second ammonia-concentration measuring unit is C2(t, NH3), the target concentration at a measurement position of the second ammonia-concentration measuring unit is C20(NH3), an arbitrary cycle is T, and a time is t, C1 (NH3) ' is C1(NH3) '=C1(NH3)x sin(t/T)/(C2(t, NH3) /C20 (NH3) +0. 5) and the measurement result acquired by the first ammonia-concentration measuring unit becomes C1(NH3)''. [0018] By controlling injection of urea water using the above equation, nitrogen oxides in flue gas can be decreased, while further decreasing ammonia in flue gas discharged from the flue gas purifying device. Further, by using sin(t/T), a state where the urea SCR catalyst adsorbs ammonia in a saturated state and a state where the urea SCR catalyst adsorbs ammonia in an unsaturated state can be repeated periodically, thereby enabling to enhance the capacity of the urea SCR catalyst as a catalyst. [0019] Advantageously, the flue gas purifying device, further includes a pretreatment nitrogen-oxide-concentration measuring unit arranged on an upstream side to the catalytic unit in a flow direction of the flue gas to measure a concentration of nitrogen oxide in flue gas flowing into the catalytic unit. The control unit controls injection of urea water by the urea-water injecting unit, such that, when it is assumed that a calculation basis of the target concentration at a measurement position of the first ammonia-concentration measuring unit is C1(NH3), the target concentration at the measurement position of the first ammonia-concentration measuring unit is C1(NH3)', the measurement result acquired by the second ammonia-concentration measuring unit is C2(t, NH3) , the target concentration at a measurement position of the second ammonia-concentration measuring unit is C20 (NH3), a measurement result acquired by the pretreatment nitrogen-oxide-concentration measuring unit is C0(t, NOx) , and a reference concentration at a measurement position of the pretreatment nitrogen-oxide-concentration measuring unit is C00(NOx), C1 (NH3)' is C1(NH3)'=C1(NH3)x(Co(t, NOx)/C00(NOx) )/C2(t, NH3)./C20(NH3)+0.5) and the measurement result acquired by the first ammonia-concentration measuring unit becomes C1(NH3)'. [0020] By controlling injection of urea water using the above equation, nitrogen oxides in flue gas can be decreased, while further decreasing ammonia in flue gas discharged from the flue gas purifying device. Further, by controlling an injection amount based on the concentration of nitrogen oxides in flue gas flowing into the catalytic unit, control can be performed according to the concentration of nitrogen oxides in flue gas flowing into the catalytic unit. [0021] Advantageously, the flue gas purifying device, further includes a post-treatment nitrogen-oxide-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 nitrogen oxide in flue gas having passed through the urea SCR catalyst. The control unit controls injection of urea water by the urea-water injecting unit, such that, when it is assumed that a calculation basis of the target concentration at a measurement position of the first ammonia-concentration measuring unit is C1(NH3), the target concentration at the measurement position of the first ammonia-concentration measuring unit is C1NH3)', the measurement result acquired by the second ammonia-concentration measuring unit is C2(t,NH3), the target concentration at a measurement position of the second ammonia-concentration measuring unit is C20(NH3), a measurement result acquired by the post-treatment nitrogen-oxide-concentration measuring unit is C2(t, NOx), and a reference concentration at a measurement position of the post-treatment nitrogen-oxide-concentration measuring unit is C20(NOx), C1(NH3)' is Cx (NH3) ' =d (NH3)x (C2 (t, NOx)/C20(NOx) )/C2(t, NH3)/C20(NH3)+0.5) and the measurement result acquired by the first ammonia-concentration measuring unit becomes C1(NH3)'. [0022] By controlling injection of urea water using the above equation, nitrogen oxides in flue gas can be decreased, while further decreasing ammonia in flue gas discharged from the flue gas purifying device. Further, by controlling the injection amount based on the concentration of nitrogen oxides in flue gas discharged from the catalytic unit, control can be performed according to a remaining amount of the of nitrogen oxides. [0023] Advantageously, the flue gas purifying device, further includes a pretreatment nitrogen-oxide-concentration measuring unit arranged on an upstream side to the catalytic unit in a flow direction of the flue gas to measure a concentration of nitrogen oxide in flue gas flowing into the catalytic unit, and a post-treatment nitrogen-oxide-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 nitrogen oxides in flue gas having passed through the urea SCR catalyst. The control unit controls injection of urea water by the urea-water injecting unit, such that, when it is assumed that a calculation basis of the target concentration at a measurement position of the first ammonia-concentration measuring unit is C1(NH3), the target concentration at the measurement position of the first ammonia-concentration measuring unit is C1 (NH3)', the measurement result acquired by the second ammonia-concentration measuring unit is C2(t, NH3) , the target concentration at a measurement position of the second ammonia-concentration measuring unit is C20 (NH3), a measurement result acquired by the pretreatment nitrogen-oxide-concentration measuring unit is Co(t, NOx), a measurement result acquired by the post-treatment nitrogen-oxide-concentration measuring unit is C2(t, NOx), and (C0(t, NOx)-C2(t, NOx)) /Co (t, NOx) is r|, C1 (NH3)' is C 1 (NH3)'=C1 (NH3)x(l/ η)/(C2(t, NH3)/C20(NH3)+0.5) and the measurement result acquired by the first ammonia-concentration measuring unit becomes C1 (NH3)'. [0024] By controlling injection of urea water using the above equation, nitrogen oxides in flue gas can be decreased, while further decreasing ammonia in flue gas discharged from the flue gas purifying device. Further, by controlling the injection amount based on a NOx removal efficiency η , control can be performed so that the NOx removal efficiency can be further increased. [0025] It is desired that the second ammonia-concentration measuring unit measures the ammonia concentration in a region where the catalytic unit is arranged. [0026] Because the second ammonia-concentration measuring unit also measures the ammonia concentration in a region where the catalytic unit is arranged, ammonia measured by the second ammonia-concentration measuring unit can be caused to react with nitrogen oxides by the catalytic unit on a downstream side to the second ammonia-concentration measuring unit. Accordingly, discharged ammonia can be further decreased. [0027] Further, it is desired to include a restoring unit that performs restoration of the urea SCR catalyst, when the ammonia concentration measured by the second ammonia-concentration measuring unit and the concentration of nitrogen oxides measured by the post-treatment nitrogen-oxide-concentration measuring unit both exceed the reference concentration, respectively. It is also desired that the restoring unit heats the urea SCR catalyst at a predetermined temperature. It is also.desired to include an isocyanic-acid-concentration measuring unit arranged between the urea-water injecting unit and the catalytic unit in a flow direction of flue gas, to measure the concentration of isocyanic acid in flue gas, and a temperature adjusting unit that adjusts the temperature of a flue-gas flow path between the urea-water injecting unit and the catalytic unit in the flow direction of the flue gas, so that the temperature adjusting unit adjusts the temperature of the flue-gas flow path based on the concentration of isocyanic acid measured by the isocyanic-acid-concentration measuring unit. Further, it is desired to include a pretreatment ammonia-concentration measuring unit arranged between the urea-water injecting unit and the catalytic unit in the flow direction of the flue gas to measure the concentration of ammonia in flue gas, and a temperature adjusting unit that adjusts the temperature of the flue-gas flow path between the urea-water injecting unit and the catalytic unit in the flow direction of the flue gas, so that the temperature adjusting unit adjusts the temperature of the flue-gas flow path based on the ammonia concentration measured by the pretreatment ammonia-concentration measuring unit. Advantageous Effects of Invention [0028] In the flue gas purifying device according to the present invention, the control unit controls injection of urea water performed by the urea-water injecting unit based on a concentration of ammonia contained in flue gas in the middle of the urea SCR catalyst and ammonia contained in flue gas having passed through the urea SCR catalyst. Accordingly, nitrogen oxides in flue gas can be decreased, while further decreasing ammonia in flue gas discharged from the flue gas purifying device. Brief Description of Drawings [0029] FIG. 1 is a block diagram of a schematic configuration of a vehicle according to an embodiment of the present invention having a flue gas purifying device according to the present invention. FIG. 2 is a block diagram of a schematic configuration of a concentration measuring unit in a flue gas purifying device for a diesel engine shown in FIG. 1. FIG. 3 is a schematic sectional view of a urea SCR catalytic unit. FIG. 4 is a block diagram of a schematic configuration of a vehicle according to another embodiment having the flue gas purifying device. FIG. 5 is a block diagram of a schematic configuration of a vehicle according to another embodiment having the flue gas purifying device. FIG. 6 is a block diagram of a schematic configuration of a vehicle according to another embodiment having the flue gas purifying device. Description of Embodiments [0030] Exemplary embodiments a flue gas purifying device according to the present invention will be explained below in detail with reference to the accompanying drawings. The present invention is not limited to the embodiments. In the following embodiments, it is assumed that an internal combustion engine mounted on the flue gas purifying device is a diesel engine, and a vehicle using the internal combustion engine is a vehicle having a diesel engine. However, the internal combustion engine is not limited thereto, and the present invention is also applicable to various internal combustion engines such as a gasoline engine and a gas turbine. Further, a device having the internal combustion engine is not limited to a vehicle, and the device can be used as an internal combustion engine of various devices such as a ship and a power generator. A burning appliance mounted on the flue gas purifying device is not limited to the internal combustion engine, and the flue gas purifying device can be also applicable to various burning appliances such as an incinerator, a pyrolytic furnace, a melting furnace, a boiler, and an external combustion engine. Waste can include various waste products. The present invention is also applicable to an incineration system that burns substances other than waste in an incinerator. [0031] FIG. 1 is a block diagram of a schematic configuration of a vehicle according to an embodiment having a diesel engine mounted on the flue gas purifying device according to the present invention. FIG. 2 is a block diagram of a schematic configuration of a concentration measuring unit in the flue gas purifying device shown in FIG. 1. As shown in FIG. 1, a vehicle 10 includes a diesel engine 12, an exhaust pipe 14 for guiding flue gas discharged from the diesel engine 12, and a flue gas purifying device 16 that purifies flue gas flowing in the exhaust pipe 14. The vehicle 10 also includes various elements required for a vehicle, such as wheels, a body, operating parts, and a transmission, other than constituent elements shown in the drawings. [0032] The diesel engine 12 is an internal combustion engine that uses light oil or heavy oil as a fuel, and burns the fuel to extract power. The exhaust pipe 14 is connected to the diesel engine 12 at one end thereof, to guide flue gas discharged from the diesel engine 12. [0033] The flue gas purifying device 16 includes an oxidation catalyst 18, a DPF 20, a urea SCR system 21, an injecting unit 22, a concentration measuring unit 28, a post-treatment ammonia-concentration measuring unit 29, and a control unit 30, and is arranged in an exhaust path of flue gas, that is, in the exhaust pipe 14 or adjacent to the exhaust pipe 14. As described later, the urea SCR system 21 includes a urea water tank 24 and a urea SCR catalytic unit 26. [0034] The oxidation catalyst 18 is a catalyst such as platinum provided in the exhaust path of flue gas, specifically, inside of a downstream portion of an exhaust port of the diesel engine 12 in a flow direction of flue gas in the exhaust pipe 14. A part of particulate matters (PM) in flue gas having passed in the exhaust pipe 14 and through the oxidation catalyst 18 is removed by the oxidation catalyst 18. The PM here is an air contaminant discharged from the diesel engine, and is a mixture of solid carbon particles, unburned hydrocarbon (Soluble Organic Fraction: SOF) formed of polymeric molecules, and sulfate generated by oxidation of sulfur contained in the fuel. The oxidation catalyst 18 oxidizes nitrogen monoxide contained in flue gas flowing in the exhaust pipe 14 to nitrogen dioxide. [0035] The DPF (Diesel Particulate Filter) 20 is a filter provided in the exhaust path of flue gas, specifically, inside of a downstream portion of the oxidation catalyst 18 in the exhaust pipe 14, to trap particulate matters contained in flue gas having passed through the oxidation catalyst 18. As the DPF 20, it is desired to use a continuous-regenerative DPF that can maintain the trapping performance such that regeneration is performed by removing trapped PM by burning or the like. [0036] A urea SCR (Selective Catalytic Reduction) system 21 is an NOx removal system that reduces nitrogen oxides (NO, NO2) contained in flue gas, and includes the injecting unit 22, the urea water tank 24, and the urea SCR catalytic unit 26. The injecting unit 22 is an injection device that injects urea water into the exhaust pipe 14, and an injection port is provided in a portion on a downstream side to the DPF 20 in the exhaust pipe 14. The injecting unit 22 injects urea water into the exhaust pipe 14 from the injection port. The urea water tank 24 stores urea water, and supplies urea water to the injecting unit 22. A replenishing port for replenishing urea water from an external device that supplies urea water is provided in the urea water tank 24, and urea water is replenished according to need from the replenishing port. The urea SCR catalytic unit 26 includes a urea SCR catalyst, which is a urea selective reduction catalyst that promotes a reaction between ammonia produced from urea with nitrogen oxides, and a support mechanism provided inside of a downstream portion of the injecting unit 22 in the exhaust pipe 14 to support the urea SCR catalyst. Zeolite catalyst can be used as the urea SCR catalyst. Further, the support mechanism is arranged inside the exhaust pipe 14, and a hole for aerating flue gas is formed therein, and the urea SCR catalyst is supported on the surface thereof. [0037] The urea SCR system 21 has the configuration described above, and injects urea water into the exhaust pipe 14 by the injecting unit 22. The injected urea water becomes ammonia (NH3) due to heat in the exhaust pipe 14. Specifically, ammonia is produced from urea water according to a chemical reaction as shown in the following equation. Thereafter, produced ammonia flows in the exhaust pipe 14 together with flue gas and reaches the urea SCR catalytic unit 26. A part of urea water is not used for producing ammonia and reaches the urea SCR catalytic unit 26 in the state of urea water. Therefore, even in the urea SCR catalytic unit 2 6, ammonia is produced from urea water according to the reaction mentioned above. Ammonia having reached the urea SCR catalytic unit 26 reacts with nitrogen oxides contained in flue gas to remove oxygen from nitrogen oxides and is reduced to nitrogen. Specifically, nitrogen oxides are reduced according to the following chemical reaction. [0038] The concentration measuring unit 28 is arranged in the urea SCR catalytic unit 26 in the exhaust path of flue gas, that is, in such a manner that an upstream face and a downstream face are both in contact with the urea SCR catalytic unit 26, to measure the ammonia concentration in flue gas flowing in the urea SCR catalytic unit 26. As shown in FIG. 2, the concentration measuring unit 28 includes a measuring unit body 40, an optical fiber 42, a measuring cell 44, and a light receiving unit 46. [0039] The measuring unit body 40 has a light emitting unit that emits laser beams in a wavelength region absorbed by ammonia, and a computing unit that calculates the ammonia concentration from a signal. The measuring unit body 40 outputs laser beams to the optical fiber 42 and receives a signal received by the light receiving unit 46. [0040] The optical fiber 42 guides laser beams output from the measuring unit body 40 so as to enter into the measuring cell 44. [0041] The measuring cell 44 is arranged in a part of the exhaust pipe 14, and includes an incident unit that causes light emitted from the optical fiber 42 to enter into the measuring cell 44, and an output unit that outputs laser beams having passed through a predetermined route in the measuring cell 44. [0042] The light receiving unit 46 receives laser beams having passed through the inside of the measuring cell 44 and output from the output unit, and outputs an intensity of received laser beams to the measuring unit body 40 as a light receiving signal. [0043] The concentration measuring unit 28 has the configuration described above, and laser beams output from the measuring unit body 40 pass through the predetermined route in the measuring cell 44 from the optical fiber 42 and is output from the output unit. At this time, if ammonia is contained in flue gas in the measuring cell 44, laser beams passing through the measuring cell 44 are absorbed. Therefore, an output of laser beams reaching the output unit changes according to the ammonia concentration in flue gas. The light receiving unit 4 6 converts laser beams output from the output unit to a light receiving signal, and outputs the light receiving signal to the measuring unit body 40. The measuring unit body 40 compares the intensity of output laser beams with an intensity calculated from the light receiving signal, to calculate the ammonia concentration in flue gas flowing in the measuring cell 44 based on its rate of diminution. Thus, the concentration measuring unit 28 uses TDLAS (Tunable Diode Laser Absorption Spectroscopy) to calculate and/or measure the ammonia concentration in flue gas passing through a predetermined position in the measuring cell 44, that is, a measurement position based on the intensity of output laser beams and the light receiving signal detected by the light receiving unit 46. The concentration measuring unit 28 according to the present embodiment can continuously calculate or measure the ammonia concentration. [0044] Only the incident unit and the output unit of the measuring cell 44 can be made of a light transmitting material, or the measuring cell 44 on the whole can be made of the light transmitting material. Further, at least two optical mirrors can be provided in the measuring cell 44, so that laser beams entering from the incident unit is multiply-reflected by the optical mirrors and output from the output unit. By multiply-reflecting laser beams, laser beams can pass through more regions in the measuring cell 44. Accordingly, an influence of concentration distribution on flue gas flowing in the measuring cell 44 can be decreased, thereby enabling accurate detection of concentrations. [0045] The post-treatment ammonia-concentration measuring unit 29 is arranged in the exhaust pipe 14 on the downstream side of the urea SCR catalytic unit 2 6 in the exhaust path of flue gas, to measure the concentration of ammonia in flue gas having passed through the urea SCR catalytic unit 26. The post-treatment ammonia-concentration measuring unit 29 is a measuring unit same as the concentration measuring unit 28, and continuously measures the ammonia concentration according to the same method as that of the concentration measuring unit 28. [0046] The control unit 30 controls the amount of urea water to be injected from the injecting unit 22 and injection timing according to PID control based on detection results acquired by the concentration measuring unit 28 and the post-treatment ammonia-concentration measuring unit 29. An example of a control method is explained below. [0047] An initial target concentration C10(NH3) of an ammonia concentration at a measurement position of the concentration measuring unit 28 and a target concentration C20(NH3) of an ammonia concentration at a measurement position of the post-treatment ammonia-concentration measuring unit 29 are set as initial values. The initial target concentration C10(NH3) and the target concentration C20(NH3) are preset values, and are stored in a storage unit of the control unit 30. The initial target concentration C10(NH3) is the ammonia concentration capable of causing a reaction between ammonia and nitrogen oxides by the urea SCR catalytic unit 2 6 in a portion on a downstream side to the concentration measuring unit 28. [0048] When the diesel engine 12 is driven and flue gas is discharged from the diesel engine 12, the control unit 30 starts control of the injecting unit 22. The control unit 30 acquires an ammonia concentration Ci(t, NH3) measured by the concentration measuring unit 28 and an ammonia concentration C2(t, NH3) measured by the post-treatment ammonia-concentration measuring unit 29. [0049] The control unit 30 calculates a new target concentration c1 ( NH3)' based on the set target concentration of the ammonia concentration and the ammonia concentration C2(t, NH3) . Specifically, the control unit 30 calculates the new target concentration C1(NH3)' by assigning respective values into C1 (NH3) ' =C1 (NH3) /C2 (t, NH3) /C20(NH3)+0.5) ••• (Equation 1). In a calculation at the time of startup, the initial target concentration C10(NH3) is assigned as the target concentration C1(NH3), and the target concentration C1(NH3)' calculated by the calculation immediately before is assigned as C1(NH3) from the next calculation. That is, the target concentration C1(NH3) is updated for each calculation. The target concentration C20 (NH3) is not updated from the initially set ammonia concentration. [0050] Next, the control unit 30 calculates an injection amount of urea water based on the calculated target concentration C1 (NH3)' and the measured ammonia concentration C1 (t, NH3) . Specifically, when C1 (NH3) ' >C 1(t, NH3), the control unit 30 increases an amount of urea water to be injected or decreases an injection interval of urea water. When C1 (NH3) '