DESCRIPTION EXHAUST GAS PURIFICATION APPARATUS AND METHOD FOR INTERNAL COMBUSTION ENGINE 5 TECHNICAL FIELD [0001] The present invention relates to an exhaust gas purification apparatus and an exhaust gas purification method for an internal combustion engine. It especially relates to an exhaust gas purification apparatus and an exhaust gas purification method for an internal combustion engine employing selective catalytic reduction (SCR), a technique for selectively 10 purifying nitrogen oxide (NOx) in exhaust gas using ammonia as a reducing agent. BACKGROUND [0002] To meet the exhaust emission regulations (NOx and PM emission regulations) for diesel engines, recent techniques reduce NOx and PM by performing various controls such as 15 hardware control, operation control, and temperature control of the engine, and also purify exhaust gas using an after-treatment device. As an after-treatment device, a diesel particulate filter (DPF) for collecting particulate matters (PM) and an SCR catalyst for purifying NOx are often used. [0003] Using an SCR catalyst, NOx is purified as follows, for instance. 20 1) Urea is injected at an upstream side of the SCR catalyst in response to the amount of NOx at the upstream side of the SCR, the temperature of the SCR catalyst, the flow rate of the exhaust gas, or the like. The amount of NOx here is measured by a sensor, or estimated. 2) The injected urea is degraded to ammonia (NH3) to be adsorbed on the SCR catalyst. 3) NOx in the exhaust gas is purified by the adsorbed NH3 when passing through the SCR 25 catalyst. [0004] Meanwhile, when the amount of NH3 exceeds an amount that can be adsorbed on the SCR catalyst, NH3 is discharged into the exhaust gas, which is a phenomenon referred to as NH3 slip. When the amount of NH3 adsorption is small, the NOx purification rate decreases. Thus, monitoring the amount of NH3 adsorption is important in determining the purification rate of NOx. However, it is difficult to measure an actual amount of NH3 adsorption, and thus the amount of NH3 adsorption is generally estimated on the basis of e.g. a measurement value of 5 NOx sensor mounted to an exhaust duct. Still, when estimating the amount of NH3 adsorption, an error inevitably occurs due to various causes. For instance, there may be measurement errors or calculation errors of various parameters due to decreases in the accuracy of various sensors, for instance. If subsequent processes are to be continued using data including such errors, the errors would be 10 accumulated, which prevents highly accurate control of the amount of urea injection and suitable purification of NOx. As a result, it becomes necessary to initialize the amount of NH3 adsorption. [0005] Patent Document 1 discloses an exhaust purification apparatus of an internal combustion engine which initializes the amount of NH3 adsorption. 15 That is, in the technique for initializing the amount of NH3 adsorption disclosed in Patent Document 1, the amount of reducing-agent adsorption on a NOx catalyst is calculated on the basis of time-series data of balance between supply of the reducing agent to the NOx catalyst upon adding the reducing agent by a reducing-agent adding means and consumption of the reducing agent in reducing reaction in the NOx catalyst. Further, the amount of NOx 20 introduced to the NOx catalyst or correlated parameters are calculated. Finally, on the basis of the calculated result, initialization is executed at a determined timing. Citation List Patent Literature 25 [0006] Patent Document 1 : Japanese Unexamined Patent Application No. 2009-28 1350 SUMMARY Technical Problem [0007] As described above, the initialization procedure in Patent Document 1 is troublesome and associated with complicated calculations. The present invention was proposed to overcome the above inconvenience. An object 5 of the present invention is to provide an exhaust gas purification apparatus and an exhaust gas purification method for an internal combustion engine capable of initializing an SCR catalyst by heating the SCR catalyst under predetermined conditions without troublesome procedures or complicated calculations. 10 Solution to Problem [0008] In order to achieve the above object, according to claim 1 of the present invention, an exhaust gas purification apparatus for an internal combustion engine includes: an SCR catalyst for purifying NOx in exhaust gas of the internal combustion engine, disposed on an exhaust-gas passage of the internal combustion engine; and an exhaust gas purification 15 controller part. The exhaust gas purification controller part includes: a NH3 adsorption amount estimate value calculation part configured to calculate an estimate value of an amount of NH3 adsorption which is adsorbed on the SCR catalyst, on the basis of an amount of injection of NH3 produced from urea-aqueous solution injected into the exhaust-gas passage from an upstream side of the SCR catalyst and an amount of NH3 for purifying NOx at the 20 upstream side of the SCR catalyst; a NOx amount estimate value derivation part configured to derive an estimate value of an amount of NOx at a downstream side of the SCR catalyst on the basis of the estimate value of the amount of NH3 adsorption from the NH3 adsorption amount estimate value calculation part and a temperature of the SCR catalyst; and a SCR catalyst heating command part configured to heat the SCR catalyst to initialize the amount of 25 NH3 adsorption of the SCR catalyst on the basis of an error between the estimate value of the amount of NOx from the NOx amount estimate value derivation part and a measurement value of the amount of NOx at the downstream side of the SCR catalyst. [0009] To initialize the SCR catalyst, the present invention focuses on the decrease in the amount of NH3 adsorption upon an increase of the temperature of the SCR catalyst. That is, the decreasing amount of NH3 adsorption upon an increase in the temperature of the SCR catalyst means that NH3 would be discharged almost directly, which is defined as initialization of the SCR catalyst. 5 Since it is difficult to obtain an actual measurement of the amount of NH3 adsorption, it is estimated from the amount of urea (NH3) injection and the amount of NH3 injection for purifying the amount of NOx at the upstream side of the SCR catalyst, as in the following equation. (Equation 1) 10 Estimate value of the amount of NH3 adsorption (g) = (the amount of urea injection - the amount of NH3 injection for purification) Then, from a map showing a relationship between the estimate value of the amount of NH3 adsorption and the temperature of the SCR catalyst, for instance, the purification rate of NOx is derived, and this purification rate of NOx and the amount of NOx at the upstream side 15 of the SCR catalyst are calculated to estimate the amount of NOx at the downstream side of the SCR catalyst. Next, this estimate value of the amount of NOx at the downstream side of the SCR catalyst is compared to the actual measurement value of the amount of NOx at the downstream side of the SCR catalyst. If the error occurring is increasing, it is determined 20 that continuing the subsequent processes with the data including such an error leads to accumulation of the errors, which makes it impossible to execute highly accurate control of the amount of urea injection and suitable purification of NOx. As a result, the SCR catalyst is heated to be initialized. [OO 101 According to the present invention described in claim 1, the estimate value of the 25 amount of NOx at the downstream side of the SCR catalyst is derived from the estimate value of the NH3 adsorption from the NH3 adsorption amount estimate value calculation part and the temperature of the SCR catalyst. The temperature of the SCR catalyst is then increased side of the SCR catalyst and the measurement value of the amount of NOx at the downstream side of the SCR catalyst. As a result, the amount of NH3 adsorption decreases. In consequence, the NH3 injected from the upstream side of the SCR catalyst is not adsorbed on the SCR catalyst so that the SCR catalyst does not execute NOx purification at 5 all, which means that initialization of the SCR catalyst is achieved. [0011] Further, according to the present invention described in claim 2, the exhaust gas purification controller part includes a SCR catalyst initialization necessity determination part configured to determine necessity of initialization of the SCR catalyst on the basis of an error between the estimate value of the amount of NOx and the measurement value of the amount 10 of NOx at the downstream side of the SCR catalyst. [0012] Further, according to the present invention described in claim 3, the SCR catalyst initialization necessity determination part is configured to determine the necessity of initialization of the SCR catalyst on the basis of whether a predetermined time has elapsed after completion of heating of the SCR catalyst. 15 [0013] According to claims 2 and 3, it is possible to issue the SCR catalyst heating commands for heating the SCR catalyst only when it is necessary to initialize the SCR catalyst. [0014] Further, according to the present invention described in claim 4, the exhaust gas purification controller part includes a SCR catalyst heating capability determination part 20 configured to determine that the heating of the SCR catalyst is possible, when it is determined that an operation is within an operation range in which the heating is possible and that forced regeneration of a filter for removing particulate matters in the exhaust gas is unnecessary. [0015] As a result, it is possible to determine that the heating of the SCR catalyst is possible when the operation is within the operation range in which the heating is possible and 25 the SCR catalyst is not heated for forced regeneration of the filter for removing particulate matters. Thus, the SCR catalyst would not be heated unnecessarily. [0016] Further, according to the present invention described in claim 5, an exhaust gas purification method is for an internal combustion engine that purifies NOx in exhaust gas in the internal combustion engine using a SCR catalyst arranged on an exhaust-gas passage of the internal combustion engine. The exhaust gas purification method includes: a NH3 adsorption amount estimate value calculation step of calculating an estimate value of an amount of NH3 adsorption which is adsorbed on the SCR catalyst, on the basis of an amount 5 of injection of NH3 produced from urea-aqueous solution injected into the exhaust-gas passage from an upstream side of the SCR catalyst and an amount of NH3 for purifying NOx at the upstream side of the SCR catalyst; a SCR catalyst initialization necessity determination step of deriving an estimate value of an amount of NOx at a downstream side of the SCR catalyst on the basis of the estimate value of the amount of NH3 adsorption and a temperature 10 of the SCR catalyst, and determining necessity of initialization of the SCR catalyst on the basis of an error between the estimate value of the amount of NOx and a measurement value of the amount of NOx at the downstream side of the SCR catalyst; and a SCR catalyst heating step of heating the SCR catalyst to initialize the amount of NH3 adsorption of the SCR catalyst. 15 [0017] In this way, it is possible to calculate the estimate value of the amount of NH3 adsorption and then derive the estimate value of the amount of NOx at the downstream side of the SCR catalyst from the estimate value of NH3 adsorption and the temperature of the SCR catalyst. Further, it is possible to obtain an error between the estimate value of the amount of 20 NOx at the downstream side of the SCR catalyst and the measurement value of the amount of NOx at the downstream side of the SCR catalyst, and then determine the necessity of initialization of the SCR catalyst from this error. When it is determined that it is necessary to initialize the SCR catalyst, the SCR catalyst is heated so that the amount of NH3 adsorption decreases. 25 As a result, NH3 injected from the upstream side of the SCR catalyst is not adsorbed on the SCR catalyst and the SCR catalyst is in a state where NOx purification cannot be performed at all, which means that initialization of the SCR catalyst is achieved. [0018] Further, according to the present invention described in claim 6, before the SCR catalyst heating step, a SCR catalyst heating capability determination step is executed. This step determines that heating of the SCR catalyst is possible when it is determined that an operation is within an operation range in which heating is possible and that forced regeneration of a filter for removing particulate matters in the exhaust gas is unnecessary. 5 [0019] In this way, it is possible to determine that the heating of the SCR catalyst is possible when the operation is within an operation range in which the heating is possible and that the SCR catalyst is not heated for the forced regeneration of a filter for removing particulate matters in the exhaust gas. As a result, the SCR catalyst is not heated unnecessarily. 10 [0020] According to the present invention described in claim 7, in the SCR catalyst initialization necessity determination step, the SCR catalyst is determined to be necessary when a cumulative value of NOx errors is smaller than a lower limit of an error, or when the cumulative value of NOx errors is greater than an upper limit of an error. [0021] In this way, it is possible to determine that the initialization of the SCR catalyst is 15 necessary when the cumulative value of NOx error is smaller than the lower limit of an error, or when the cumulative value of NOx errors is greater than the upper limit of an error. [0022] Further, according to the present invention described in claim 8, in the SCR catalyst initialization necessity determination step, the SCR catalyst is determined to be necessary when a predetermined time has elapsed after determining completion of the heating 20 of the SCR catalyst. [0023] In this way, when a predetermined time has elapsed after determining completion of the heating of the SCR catalyst, it may be considered that purification by the SCR catalyst is considerably executed so that the NOx errors has been accumulated, and to determine that the initialization of the SCR catalyst is necessary. 25 [0024] Further, according the present invention described in claim 9, in the SCR catalyst heating step, an inlet temperature of a filter for removing particulate matters that collects particulate matters in the exhaust gas at the upstream side of the SCR catalyst is controlled by a control used upon forced regeneration of the filter for removing particulate matters. [0025] In this way, it is possible to simplify the control of the inlet temperature of the filter for removing PM. [0026] Further, according to the present invention described in claim 10, the inlet temperature of the filter for removing particulate matters is controlled on the basis of a target 5 inlet temperature that is different from one used upon forced regeneration of the filter for removing particulate matters. [0027] In this way, it is possible to restrict the inlet temperature of the filter for removing PM to be low, and thus to reduce oil dilution. [0028] Further, according to the present invention described in claim 11, the inlet 10 temperature is increased to the target inlet temperature at a predetermined increase rate from a start of temperature increase. [0029] In this way, it is possible to restrict NH3 slip. [0030] Further, according the present invention described in claim 12, the inlet temperature is increased to the target inlet temperature in a plurality of stages from the start of 15 the temperature increase. [003 11 In this way, it is possible to reduce NH3 slip and oil dilution. [0032] Further, according to the present invention described in claim 13, the target inlet temperature is corrected by the temperature of the SCR catalyst or a temperature at the upstream side of the SCR catalyst. 20 [0033] Further, according to the present invention described in claim 14, the inlet temperature of the filter for removing particulate matters is controlled on the basis of an outlet temperature of the filter for removing particulate matters. [0034] Further, according to the present invention described in claim 15, the inlet temperature of the filter for removing particulate matters is controlled on the basis of the 25 temperature at the upstream side of the SCR catalyst. [0035] Further, according to the present invention described in claim 16, the inlet temperature of the filter for removing particulate matters is controlled on the basis of the temperature of the SCR catalyst. [0036] According to the above claims 12 to 16, it is possible to heat the SCR catalyst securely even in a cold place. Advantageous Effects 5 [0037] According to the present invention, it is possible to achieve initialization of the SCR catalyst by calculating the estimate value of the amount of NH3 adsorption, deriving the estimate value of the amount of NOx at the downstream side of the SCR catalyst, determining the necessity of the initialization of the SCR catalyst on the basis of the error between the estimate value of the amount of NOx at the downstream side of the SCR catalyst and the 10 measurement value of the amount of NOx at the downstream side of the SCR catalyst, and heating the SCR catalyst. Thus, it is no longer necessary to execute the troublesome steps and complicated calculations, unlike the conventional techniques. BRIEF DESCRIPTION OF DRAWINGS 15 [0038] FIG. 1 is a schematic system diagram according to the first embodiment of an internal combustion engine for implementing an exhaust gas purification method for an internal combustion engine according to the present invention. FIG 2 is a partial configuration diagram schematically illustrating an exhaust gas purification apparatus in the internal combustion engine of FIG. 1. 20 FIG. 3 is a flowchart of an SCR catalyst initializing process according to the first embodiment. FIG. 4 is a graph of the amount of NH3 that can be adsorbed, related to a temperature of the SCR catalyst. FIG. 5 is a graph of a time-series change comparing the estimated value and the actual 25 value of the amount of NH3 adsorption, and of a corresponding change in the temperature of the SCR catalyst. FIG. 6 is a graph of an example of an increase at a predetermined rate of the temperature of the SCR catalyst to a target temperature from a DPF inlet temperature at execution of the control of the DPF inlet temperature, upon increasing the temperature of the SCR catalyst in the second embodiment. FIG. 7 is a graph of an example of a two-stage increase of the temperature to the target temperature from the DPF inlet temperature. 5 FIG. 8 is a graph of an example of a multi-stage increase of the temperature to the target temperature from the DPF inlet temperature. FIG. 9 is a schematic graph of correction of a DPF inlet target temperature according to the third embodiment. FIG. 10 is a block diagram of a flow of a specific correcting step for correcting the DPF 10 inlet target temperature as illustrated in FIG. 9. DETAILED DESCRIPTION [0039] An exhaust gas purification apparatus and an exhaust gas purification method for an internal combustion engine according to the present embodiment will be described below 15 in detail listing various embodiments and referring to the accompanying drawings. [0040] (First embodiment) FIG. 1 illustrates an example of an intake-and-exhaust system and an electrical system of an internal combustion engine 1 including an exhaust gas purification apparatus of the first embodiment. 20 The internal combustion engine 1 includes an intake-and-exhaust system in which an intake duct 3, an exhaust duct 4, and an EGR duct 5 are connected to an engine body 2. A compressor and a turbine of a turbocharger 6 are disposed on the flow path of the exhaust duct 4 so as to be interposed between the upstream side of the intake duct 3 and the vicinity of the outlet of the engine. An intercooler 7 is disposed on the intake duct 3. Further, the intake 25 duct 3 includes an intake throttle valve 8, while the EGR duct 5 includes an EGR valve 9. Moreover, an exhaust gas purification apparatus serving as an after-treatment device for exhaust gas is connected to the exhaust duct 4 so as to communicate with the exhaust duct 4. [0041] An electronic control unit (ECU, i.e., engine control unit) is provided for the intake-and-exhaust system of the above engine body 2, as a part constituting the electrical system. The ECU 11 receives sensor outputs from various sensors (described below) disposed on the engine body 2, the intake duct 3, the exhaust duct 4, the EGR duct 5, and the exhaust gas purification apparatus 10. The ECU 11 also receives acceleration input signals 5 and signals from DCU, which will be described below. In addition to executing the predetermined signal processing and calculation processing, the ECU 11 executes the essentials of the present invention, such as a step for determining the SCR catalyst initialization, determination of whether it is possible to heat the SCR catalyst, and heating of the SCR catalyst, which will be described below. 10 An in-vehicle battery 13 is electrically connected to the ECU 11 via a key switch Sw. A starter motor MSt is electrically connected to the in-vehicle battery 13 via the key switch Sw. [0042] An injector, a common-rail pressure sensor, a combustion temperature sensor, a crank sensor, a cam sensor, a water temperature sensor, a hydraulic pressure switch, or the like, none of which is illustrated, are connected to the engine body 2 to communicate signals with 15 theECU11. An airflow meter m,f and an intake temperature sensor S,nt are disposed at the upstream side of the intake duct 3. The intake throttle valve 8, an intake absolute pressure sensor S,, and an intake temperature sensor Slnta re disposed on the downstream side of the intake duct 3, which is the inlet side of the engine body 2. 20 The EGR valve 9 is disposed on the EGR duct 5. A DOC inlet temperature sensor Sdolnt, a DPF inlet temperature sensor Sdplnt, a DPF differential pressure sensor Sdp,a DPF outlet temperature sensor Sdpouat, NOx sensor SnI,a n exhaust temperature sensor (not illustrated), a NOx sensor S ~aZre each disposed on the exhaust gas purification apparatus 10 at the downstream side of the exhaust duct 4, arranged 25 in this order from the upstream side. The details will be described later. The signals of the NOx sensor S,,, the exhaust temperature sensor St, and the NOx sensor Snz are transmitted the ECU 11. Further, the turbocharger 6 and the intake throttle valve 8 are driven by the ECU 11. [0043] Next, in reference to FIG. 2 further illustrating the exhaust gas purification apparatus 10 in the above internal combustion engine 1, the essential configuration of the present invention will be described in detail. The configuration around the engine body 2 is as described above, and thus not explained herein. 5 The exhaust gas purification apparatus 10 first includes an oxidation catalyst 20, which is a diesel oxidation catalyst (DOC) herein, disposed on the upstream side of the exhaust duct 4 constituting the exhaust-gas passage connected to the engine body 2. The exhaust gas purification apparatus 10 also includes a SCR catalyst 21 for purifying NOx in the exhaust gas of the engine body 2, disposed at the downstream side of the oxidation catalyst 20, and 10 another oxidation catalyst 22 for removing excess ammonia discharged from the SCR catalyst 21, disposed at the downstream side of the SCR catalyst 21. Further, a diesel particulate filter (DPF) 23 may be disposed between the oxidation catalyst 20 and the SCR catalyst 21 at the downstream side of the oxidation catalyst 20. The DPF 23 is a filter for removing particulate matters (PM) that collects PM in the exhaust gas. In FIG. 1, the DPF 23 and the 15 DOC 20 are integrally accommodated. A urea injection unit 24 for injecting urea-aqueous solution is disposed at the downstream side of the DPF 23, which is also the upstream side of the SCR catalyst 21. Although not illustrated in detail, the urea injection unit 24 includes an electromagnetic injection valve 24v and a tank 24t for storing urea-aqueous solution. The electromagnetic 20 injection valve 24v has an injection nozzle disposed inside the exhaust duct 4. When the urea injection unit 24 injects urea-aqueous solution into the exhaust duct 4 I through the electromagnetic injection valve 24v, chemical reaction that begins with evaporation caused by exhaust heat and leads to hydrolysis produces ammonia NH3 which serves as a direct reducing agent. NH3 and NOx in the exhaust gas react while passing I 25 through the SCR catalyst 21 to change into nitrogen N2 and water H20. With this reaction, purification of NOx is executed. Further, in this case, detection signals from the NOx sensors S,,, Sn2 disposed on the upstream and downstream sides, respectively, of the SCR catalyst 21 are transmitted to the ECU 11 through CAN, so that the amount of injection of the urea-aqueous solution is controlled in accordance with the important engine parameters such as the operation temperature and the engine speed. In the above internal combustion engine 1, the ECU 11 executes steps such as the 5 predetermined signal processing and calculation processing, and the NOx purifying processing of the exhaust gas purification apparatus 10 in response to sensor outputs from the various sensors disposed on the engine body 2, the intake duct 3, the exhaust duct 4, the EGR duct 5, and the exhaust gas purification apparatus 10, and to the acceleration input signals. [0044] The ECU 11 includes an exhaust gas purification controller part 11C for executing 10 the steps of the NOx purification process. The exhaust gas purification controller part 11C includes a NH3 adsorption amount estimate value calculation part A, a NOx amount estimate value derivation part B, a SCR catalyst heating command part C, a SCR catalyst initialization necessity determination part D, a SCR catalyst heating capability determination part E, and a SCR catalyst heating completion 15 determination part F. The NH3 adsorption amount estimate value calculation part A calculates an estimate value of the amount of NH3 adsorption that is adsorbed on the SCR catalyst, on the basis of the amount of NH3 injection produced from urea-aqueous solution injected into the exhaustgas passage 4 from the upstream side of the SCR catalyst, and the amount of NH3 injection 20 for purifying NOx at the upstream side of the SCR catalyst. The NOx amount estimate value derivation part B derives an estimate value of the amount of NOx at the downstream side of the SCR catalyst on the basis of the estimate value of the amount of NH3 adsorption from the NH3 adsorption amount estimate value calculation part, and the temperature of the SCR catalyst. 25 The SCR catalyst heating command part C initializes the SCR catalyst by heating the SCR catalyst to initialize the estimate value of the NH3 adsorption, on the basis of an error between the estimate value of the amount of NOx from the NOx amount estimate value derivation part and a measurement value of the amount of NOx at the downstream side of the SCR catalyst. The SCR catalyst initialization necessity determination part D determines necessity of initialization of the SCR catalyst, on the basis of the error between the estimate value of the amount of NOx and the measurement value of the amount of NOx at the downstream side of 5 the SCR catalyst, on the basis of whether a predetermined time has elapsed after completing heating of the SCR catalyst, or on the basis of the measurement value of the amount of NOx at the downstream side of the SCR catalyst. The SCR catalyst heating capability determination part E determines that it is possible to heat the SCR catalyst when forced regeneration of the filter for removing the particulate 10 matters in the exhaust gas is unnecessary and the operation is within an operation range in which the heating is possible. Further, the exhaust gas purification controller part includes the SCR catalyst heating completion determination part F. [0045] The first embodiment of the exhaust gas purification apparatus for implementing 15 the exhaust gas purification method for an internal combustion engine according to the present invention is as described above. Next, the steps of the NOx purification process of the exhaust gas purification apparatus 10 will be described along with a series of operations of the above internal combustion engine 1. The series of operations of the internal combustion engine 1 will be schematically described because it is not an essential part of the present 20 invention. When the key switch Sw is turned on, electric current is applied to the ECU 11 from the in-vehicle battery 13, so that the starter motor Mst is driven to rotate the crank shaft of the engine body 2. As a result, fuel is injected into the cylinder through the injector in response to a command from the ECU 11, thereby starting the engine. At this point, the combustion 25 air drawn in through the intake duct 3 is changed into air having a high pressure and a high temperature by a compressor of the turbocharger 6, and then cooled by the intercooler 7 to be supplied into the cylinder inside the engine body 2, while high-pressure fuel is injected into the cylinder through the common rail Cr and the fuel injection valve Vf. As a result, combustion is started. The combustion gas is discharged into the exhaust gas purification apparatus 10 through the exhaust duct 4 while rotating the turbine of the turbocharger 6. Further, a part of the exhaust gas is re-circulated into the cylinder of the engine body 2 through the EGR duct 5 to 5 be combusted again. [0046] Exhaust gas that has been supplied to the exhaust gas purification apparatus 10 through the exhaust duct 4 passes through the DOC 20 being an oxidation catalyst and DPF 23 in sequence, where the non-combusted portion of the PM in the exhaust gas is combusted and the PM is removed, and then passes through the SCR catalyst 21. As a result, NOx in 10 the exhaust gas is purified. Further, excess ammonia discharged from the SCR catalyst 21 is removed by the oxidation catalyst 22 at the downstream side of the SCR catalyst 21, before the exhaust gas is finally discharged. When the exhaust gas passes through the SCR catalyst 21, urea-aqueous solution is injected into the exhaust duct 4 through the electromagnetic injection valve 24v at the urea injection unit 24, so that ammonia NH3 serving as a direct 15 reducing agent is produced by chemical reaction that begins from evaporation caused by exhaust heat and leads to hydrolysis. NH3 and NOx in the exhaust gas reacts while passing through the SCR catalyst 21 and then changes into nitrogen N2 and water H20. This reaction makes it possible to purify NOx. [0047] When exhaust gas passes through the DOC 20 and the DPF 23 of the exhaust gas 20 purification apparatus 10, the temperature of the exhaust gas is detected by the DOC inlet temperature sensor Sdpintth, e DPF inlet temperature sensor Sdpintth, e DPF differential pressure sensor Sdp,a nd the DPF outlet pressure sensor Sdpoutarn d then transmitted to the ECU 11 in sequence. Next, when the exhaust gas passes through the SCR catalyst 21, detection signals from NOx sensor Snl, the exhaust temperature sensor, and the NOx sensor Sn2 are transmitted 25 to the ECU 11. [0048] The exhaust gas purification controller part 11C of the ECU 11 executes the processing steps illustrated in FIG. 3 on the basis of the received detection signals. Specifically, it executes (1) SCR catalyst initialization determination step, (2) SCR catalyst heating capability determination, (3) SCR catalyst heating, and (4) SCR catalyst heating completion determination step. First, the NH3 adsorption amount estimate value calculation part A calculates the amount of NH3 adsorption (step Sl). As described above, the amount of NH3 that can be 5 adsorbed on the SCR catalyst varies depending on the temperature of the SCR catalyst. For instance, FIG. 4 illustrates a relationship between the temperature of the SCR catalyst and the amount of NH 3 that can be adsorbed. Specifically, the amount of NH3 that can be adsorbed reaches its peak at a predetermined temperature (approximately 200 "C) and then declines after exceeding this predetermined temperature. As the temperature rises higher, the amount 10 of adsorption approaches zero, where NH3 is hardly adsorbed. The amount of NH3 adsorption can be obtained by the following equation. Estimate value of the amount of NH3 adsorption (g) = 5 (the amount of urea injection - the amount of NH3 injection for purification) The amount of urea injection here is the amount of NH3 injected from the urea injection 15 unit 24 at the upstream side of the SCR catalyst 21, while the amount of NH3 injection for purification here is the amount of NH3 required for purification. These amounts are obtained by predetermined calculations. At the same time, the NOx amount estimate value derivation part B derives an estimate value of the amount of NOx at the downstream side of the SCR catalyst, on the basis of the 20 estimate value of the NH3 adsorption from the NH3 adsorption amount estimate value calculation part A, and the temperature of the SCR catalyst. The above estimate value of the amount of NOx corresponds to the estimate value of the NH3 adsorption, and thus can be represented by a curve of the estimate value of the NH3 adsorption as indicated by the dotted line in FIG. 5. 25 On the other hand, the measurement value of the amount of NOx at the downstream side of the SCR catalyst is assumed as being the actual amount of NH3 adsorption, as indicated by the solid line. FIG. 5 also illustrates the temperature of the SCR catalyst. FIG. 5, as described above, illustrates that the difference increases between the estimate value of the amount of NH3 adsorption, which is the estimate value of the NOx amount at the downstream side of the SCR catalyst, and the actual amount of NH3 adsorption, which is the measurement value of the amount of NOx at the downstream side of the SCR catalyst. [0049] Next, in step S2, the exhaust gas purification controller part 11C of the ECU 11 5 executes the step of determining necessity of the SCR catalyst initialization. Note that the SCR catalyst initialization is determined to be necessary when one of the conditions (1) to (3) below is met. Specifically, the SCR catalyst initialization necessity determination part D determines the following conditions. (1) When an error of NOx at the outlet of the SCR catalyst is large 10 This means that, as illustrated in FIG. 5, as a result of purification of the exhaust gas having proceeded, the difference between the estimate ,value of the NH3 adsorption, which is the estimate value of the amount of NOx at the downstream side of the SCR catalyst, and the actual amount of NH3 adsorption, which is the measurement value of the amount of NOx at the downstream side of the SCR catalyst, has been accumulated, and then the error of NOx at 15 the outlet of the SCR catalyst has exceeded a predetermined threshold value. Specifically, the lower limit of the error > the cumulative value of the NOx error, or the upper limit of the error