A method for treating combustion exhaust gas from a boiler which contains NO x, SO x, mercury, CO and incompletely combusted hydrocarbons, the method comprising the steps of: a) adding a mercury-halogenating agent and ammonia to the combustion exhaust gas; b) subjecting said combustion exhaust gas to reduction denitration simultaneously with oxidizing metallic mercury to halogenated mercury in the presence of a solid catalyst; and c) wet-desulfurizing said combustion exhaust gas subjected to said step b) of reduction denitration with an alkaline absorbing solution, and removing the halogenated mercury with the alkaline absorbing solution; the method further comprising, prior to said step b) of reduction denitration, a step of: d) bringing said combustion exhaust gas into contact with a CO/incompletely combusted hydrocarbons oxidation catalyst, wherein the CO/HC oxidation catalyst is a catalyst comprising: at least one selected from the group consisting of TiO 2, SiO 2, ZrO 2, Al 2 O 3 and zeolite as a support; and at least one selected from the group consisting of Pt, Ru, Rh, Pd, Ir, Au, Ag, V, W, Mo, Ni, Co, Fe, Cr, Cu and Mn as an active component, wherein the active component is supported on the support, and wherein the composition of V, W, Mo, Ni, Co, Fe, Cr, Cu and Mn of the active component is 0.5 to 20 parts by weight as an oxide relative to 100 parts by weight of the support, and the composition of Pt, Ru, Rh, Pd, Ir, Au and Ag of the active component is 0.01 to 2 parts by weight as a metal relative to 100 parts by weight of the support, and wherein the solid catalyst is a catalyst comprising: at least one selected from the group consisting of TiO 2, SiO 2 ZrO 2 and zeolite as a support; and at least one selected from the group consisting of V, W, and Mo as an active component, wherein the active component is supported on the support, characterized in that said step d) of catalytic oxidation of CO/incompletely combusted hydrocarbons is carried out prior to said step a) of addition of said mercury-halogenating agent and ammonia. a) adding a mercury-halogenating agent and ammonia to the combustion exhaust gas; b) subjecting said combustion exhaust gas to reduction denitration simultaneously with oxidizing metallic mercury to halogenated mercury in the presence of a solid catalyst; and c) wet-desulfurizing said combustion exhaust gas subjected to said step b) of reduction denitration with an alkaline absorbing solution, and removing the halogenated mercury with the alkaline absorbing solution; d) bringing said combustion exhaust gas into contact with a CO/incompletely combusted hydrocarbons oxidation catalyst, wherein the CO/HC oxidation catalyst is a catalyst comprising: at least one selected from the group consisting of TiO 2, SiO 2, ZrO 2, Al 2 O 3 and zeolite as a support; and at least one selected from the group consisting of Pt, Ru, Rh, Pd, Ir, Au, Ag, V, W, Mo, Ni, Co, Fe, Cr, Cu and Mn as an active component, wherein the active component is supported on the support, and wherein the composition of V, W, Mo, Ni, Co, Fe, Cr, Cu and Mn of the active component is 0.5 to 20 parts by weight as an oxide relative to 100 parts by weight of the support, and the composition of Pt, Ru, Rh, Pd, Ir, Au and Ag of the active component is 0.01 to 2 parts by weight as a metal relative to 100 parts by weight of the support, and wherein the solid catalyst is a catalyst comprising: at least one selected from the group consisting of TiO 2, SiO 2 ZrO 2 and zeolite as a support; and at least one selected from the group consisting of V, W, and Mo as an active component, wherein the active component is supported on the support, characterized in that said step d) of catalytic oxidation of CO/incompletely combusted hydrocarbons is carried out prior to said step a) of addition of said mercury-halogenating agent and ammonia. at least one selected from the group consisting of TiO 2, SiO 2, ZrO 2, Al 2 O 3 and zeolite as a support; and at least one selected from the group consisting of Pt, Ru, Rh, Pd, Ir, Au, Ag, V, W, Mo, Ni, Co, Fe, Cr, Cu and Mn as an active component, wherein the active component is supported on the support, and wherein the composition of V, W, Mo, Ni, Co, Fe, Cr, Cu and Mn of the active component is 0.5 to 20 parts by weight as an oxide relative to 100 parts by weight of the support, and the composition of Pt, Ru, Rh, Pd, Ir, Au and Ag of the active component is 0.01 to 2 parts by weight as a metal relative to 100 parts by weight of the support, and wherein the solid catalyst is a catalyst comprising: at least one selected from the group consisting of TiO 2, SiO 2 ZrO 2 and zeolite as a support; and at least one selected from the group consisting of V, W, and Mo as an active component, wherein the active component is supported on the support, characterized in that said step d) of catalytic oxidation of CO/incompletely combusted hydrocarbons is carried out prior to said step a) of addition of said mercury-halogenating agent and ammonia. at least one selected from the group consisting of TiO 2, SiO 2 ZrO 2 and zeolite as a support; and at least one selected from the group consisting of V, W, and Mo as an active component, wherein the active component is supported on the support, characterized in that said step d) of catalytic oxidation of CO/incompletely combusted hydrocarbons is carried out prior to said step a) of addition of said mercury-halogenating agent and ammonia.