1. A flue gas treatment method comprising the steps of:
adding a 3C-5C olefinic hydrocarbon (unsaturated hydrocarbon) to a combustion flue gas including SO3 as well as NOx as a first additive; and then,
bringing the combustion flue gas into contact with a catalyst which includes an oxide constituted by one or more of elements selected from the group consisting of Ti, Si, Zr, and Ce and/or a mixed oxide and/or a complex oxide constituted by two or more of elements selected from the group as a carrier and which does not include a noble metal, and thereby SO3 is treated by reduction into SO2.
2. The flue gas treatment method according to Claim 1, wherein the 3C-5C olefinic hydrocarbon (unsaturated hydrocarbon) is one or more selected from the group consisting of C3H6, C4H8, and C5H10.
3. The flue gas treatment method according to Claim 2, wherein the C4H8 and C5H10 are a geometric isomer or a racemic body of either one of them.
4. The flue gas treatment method according to any one of Claims 1 to 3, wherein the carrier includes a mixed oxide and/or a complex oxide including one or more selected from the group consisting of TiO2-SiO2, TiO2-ZrO2, and TiO2-CeO2.
5. The flue gas treatment method according to any one of Claims 1 to 4, wherein the

catalyst is a catalyst in which a metal oxide including one or more selected from the group consisting of V2O5, WO3, MoO3, Mn2O3, MnO2, NiO, and Co3O4 is carried on the complex oxide as the carrier.
6. The flue gas treatment method according to Claim 5, wherein a metallosilicate-base complex oxide, in which at least a part of Al and/or Si in a zeolite crystal structure is substituted with one or more selected from the group consisting of Ti, V, Mn, Fe, and Co, is coated onto the catalyst.
7. The flue gas treatment method according to any one of Claims 1 to 6, wherein a treatment for reducing SO3 into SO2 is performed in a temperature range of 250ºC to 450ºC .
8. The flue gas treatment method according to Claim 7, wherein a treatment for reducing SO3 into SO2 is performed in a temperature range of 300ºC to 400ºC.
9. An SO3 reduction apparatus comprising:
a first injection device configured to obtain add a first additive to a combustion flue gas containing SO3 as well as NOx; and
a catalyst layer including a catalyst through which the combustion flue gas is allowed to flow,
wherein the first additive is a 3C-5C olefinic hydrocarbon (unsaturated hydrocarbon),
wherein the catalyst does not include a noble metal and includes an oxide including one or more of elements selected from the group consisting of Ti, Si, Zr, and Ce and/or a

mixed oxide and/or a complex oxide including two or more of elements selected from the group as a carrier, and
wherein the SO3 reduction apparatus is configured to perform a treatment for reducing SO3 to SO2.
10. The SO3 reduction apparatus according to Claim 9, wherein the 3C-5C olefinic hydrocarbon (unsaturated hydrocarbon) is one or more selected from the group consisting of C3H6, C4H8, and C5H10.
11. The SO3 reduction apparatus according to Claim 10, wherein the C4H8 and C5H10 are a geometric isomer or a racemic body of either one thereof.
12. The SO3 reduction apparatus according to any one of Claims 9 to 11, wherein the carrier includes a mixed oxide and/or a complex oxide including one or more selected from the group consisting of TiO2-SiO2, TiO2-ZrO2, and TiO2-CeO2.
13. The SO3 reduction apparatus according to any one of Claims 9 to 12, wherein the catalyst is a catalyst in which a metal oxide including one or more selected from the group consisting of V2O5, WO3, MoO3, Mn2O3, MnO2, NiO, and Co3O4 is carried on the complex oxide as the carrier.
14. The SO3 reduction apparatus according to any one of Claims 9 to 13, wherein the catalyst layer includes:

a first catalyst layer arranged on a back stream side of the first injection device and configured to reduce the concentration of SO3; and
a second catalyst layer arranged on a back stream side of a second injection device arranged close to the first injection device and configured to add NH3 to the combustion flue gas as a second additive, the second catalyst layer being configured to perform denitration, and
wherein the first catalyst layer is arranged on a front stream side or a back stream side of the second catalyst layer.