A Ti02-W03-V20s-based exhaust gas treatment catalyst for denitration having a honeycomb shape (height: 150 mm, breadth: 150 mm, length: 800 mm, wall thickness: 1.15 mm, pitch (distance between the centers of adjacent walls): 7.4 mm, number of meshes (n): 20><20) was used in an exhaust gas line of a coal-fired boiler for approximately 70000 hours (percentage of clogged holes: approximately 3%). The catalyst was crushed with a crusher. The fragments obtained by crushing were screened with sieves respectively having mesh sizes of 0.074 mm, 0.105 mm, 0.150 mm, 0.212 mm, 0.297 mm, 0.5 mm, and 1.0 mm (the nominal dimensions are specified according to Japanese Industrial Standards (JIS)). Then, the weight percentages and composition percentages of the fine particles passed through the sieves and of the coarse pieces left on the sieves were obtained respectively. Tables 1 and 2 below show the result. Note that, for comparison, the composition percentages of a newly-made exhaust gas treatment catalyst are also shown in Tables 1 and 2 below. As seen from Tables 1 and 2 above, it was confirmed that the mesh sizes (threshold size S) set in a range of 0.105 to 1.0 mm were able to make the weight percentage of the coarse pieces in a range of 70 to 95%. Catalysts for treating exhaust gas were regenerated by using the coarse pieces screened with the sieves having mesh sizes of 0.5 mm and 0.074 mm, respectively, in Example 1 above. Specifically, each of the coarse pieces was pulverized (to have an average particle diameter of 20 urn) with a hammermill. The fine powder thus obtained (13 kg), an organic binder (0.9 kg), and water (adequate amount) were kneaded with a kneader and uniformly mixed. The obtained kneaded product was supplied into an extruder to prepare a precursor of an exhaust gas treatment catalyst the catalyst having a honeycomb shape (height: 69 mm, breadth: 69 mm, length: 800 mm, mesh pitch: 7.4 mm, mesh opening: 6.25 mm, number of meshes (n): 9x9). The precursor was sufficiently naturally dried and subsequently dried with hot air (100°C * 5 hours). Thereafter, the dried precursor was subjected to a calcining treatment (500°C * 3 hours) in a calcining furnace, and then cut into pieces (number of meshes: 6x7). In this manner, obtained were test samples 1 (screened product with the 0.5-mm mesh) and test samples 2 (screened product with the 0.074-mm mesh) of the regenerated exhaust gas treatment catalyst (two in each test sample). Subsequently, under conditions described below, the denitration rates and SO2 oxidation rates of the respective test samples 1 and 2 were obtained, and the AS2O3 contents thereof were also obtained. Moreover, for comparison, prepared were: an exhaust gas treatment catalyst (test sample 3) regenerated under the same conditions as those of the test samples 1 and 2 after mixing coarse pieces obtained under the conditions of the Example 1, except the omission of the screening (separation step), i.e., without removing fly ash; an exhaust gas treatment catalyst (test sample 4) before the regeneration treatment; and a newly-made exhaust gas treatment catalyst (test sample 5). The denization rates and SO2 oxidation rates thereof were obtained, and the AS2O3 contents thereof were also obtained. Table 3 below shows the result. Note that, the denitration rate and the SO2 oxidation rate were calculated according to equations described below. • Test conditions • Compositions of exhaust gas- NOx. 150 ppm NH3: 150 ppm SO2 : 800 ppm O2 : 4% CO2: approximately 12.5% H2O: approximately 11.5% N2: balance • Temperature of exhaust gas: 380°C • Amount of exhaust gas: 19.97 Nm3/hr • Ugs: 2.3 Nm/sec • AV: 11.63 N3/m2-hr • Denitration rate (%)= {l-(NOx concentration at catalyst outlet / NOx concentration at catalyst inlet)} *100 • SO2 oxidation rate (%)= {(SO3 concentration at catalyst outlet -SO3 concentration at catalyst inlet) / SO2 concentration at catalyst inlet} * 100 As seen from Table 3 above, the test sample 2 (screened product with the 0.074-mm mesh) and the test sample 3 (product without screening) had improved denization rates compared with that of the test sample 4 (used product having fly ash adhered thereto), but had greatly increased SOz oxidation rates more than those of the test sample 4 (used product having fly ash adhered thereto) and the test sample 5(newly-made product). In contrast, the test sample 1 (screened product with the 0.5-mm mesh) had an improved denitration rate compared with that of the test sample 4 (used product having fly ash adhered thereto), and also had a SO2 oxidation rate suppressed to a level approximately equivalent to that of the test sample 5 (newly-made product). The SO2 oxidation rate of the test sample 1 was satisfactorily reduced compared with those of the test sample 2 (screened product with the 0.074-mm mesh) and the test sample 3 (product without screening). Moreover, the test sample 1 (screened product with the 0.5-mm mesh) can be recognized as a regenerated product made of recycled raw material because the test sample 1 contains approximately the same amount of AS2O3, which was originated from the fly ash and so forth, as those of the other test samples 2 to 4. However, the test sample 1 demonstrated comparative performances to those of the test sample 5 that was a newly-made product containing no AS2O3. From the results described above, it was confirmed that the method of regenerating an exhaust gas treatment catalyst according to the present invention is capable of regenerating an exhaust gas treatment catalyst in which the lowering of the exhaust-gas treating performance and the oxidation of sulfur dioxide are suppress. It was also confirmed that the exhaust gas treatment catalyst according to the present invention is capable of demonstrating performances that are approximately equivalent to those of a newly-made exhaust gas treatment catalyst. INDUSTRIAL APPLICABILITY A method of regenerating an exhaust gas treatment catalyst according to the present invention and an exhaust gas treatment catalyst obtained by the method are extremely useful and beneficial in various industries, since such a method and an exhaust gas treatment catalyst are capable of suppressing the lowering of the exhaust-gas treating performance and the oxidation of sulfur dioxide, and demonstrating performances approximately equivalent to those of a newly-made product. WE CLAIM: 1. A method of regenerating an exhaust gas treatment catalyst having ash adhered to a surface thereof, the method characterized by comprising: a crushing step of crushing the exhaust gas treatment catalyst that has been used; a separating step of separating the crushed exhaust gas treatment catalyst into coarse pieces having a size exceeding a threshold size S and fine particles having a size not larger than the threshold size S; a pulverizing step of pulverizing the separated coarse pieces into a fine powder; a molding step of molding the pulverized fine powder as a raw material into an exhaust gas treatment catalyst; and a calcining step of calcining a molded precursor of the exhaust gas treatment catalyst, the method characterized in that the threshold size S has a value not smaller than 0.105 mm. 2. The method of regenerating an exhaust gas treatment catalyst according to claim 1, characterized in that the crushing step is a step in which the exhaust gas treatment catalyst that has been used is crushed such that 70 to 95 wt% of the whole exhaust gas treatment catalyst that has been used becomes the coarse pieces having a size exceeding the threshold size S. 3. The method of regenerating an exhaust gas treatment catalyst according to claim 2, characterized in that the threshold size S has a value not larger than 1.0 mm. 4. The method of regenerating an exhaust gas treatment catalyst according to any one of claims 1 to 3, characterized in that the pulverizing step is a step in which the coarse pieces is pulverized such that the fine powder has an average particle diameter not larger than 0.1 mm. 5. The method of regenerating an exhaust gas treatment catalyst according to any one of claims 1 to 4, characterized in that the exhaust gas treatment catalyst is used to treat exhaust gas from a burned coal. 6. The method of regenerating an exhaust gas treatment catalyst according to claim 5, characterized in that the exhaust gas treatment catalyst is used to treat nitrogen oxide in the exhaust gas. 7. An exhaust gas treatment catalyst characterized by being regenerated by the method of regenerating an exhaust gas treatment catalyst according to any one of claims 1 to 6.