A CO 2 or H 2 S reducing system (10A, 10B, 10C, 10D) comprising at least first and second CO 2 or H 2 S reducing apparatuses (11-1, 11-2) each of which includes: an absorber (1006-1, 1006-2) that reduces at least one of CO 2 and H 2 S by bringing a provided flue gas including at least one of CO 2 and H 2 S into contact with an absorbing solution (1005-1, 1005-2), a regenerator (1008-1, 1008-2) that regenerates a rich solution (1007-1, 1007-2) that has absorbed at least one of CO 2 and H 2 S, a rich-solution supply line (12-1, 12-2) that feeds the rich solution (1007-1, 1007-2) from the absorber (1006-1, 1006-2) to the regenerator (1008-1, 1008-2), and a lean-solution supply line (13-1, 13-2) that feeds a lean solution (1009-1, 1009-2) from which at least one of CO 2 and H 2 S are reduced in the regenerator (1008-1, 1008-2), from the regenerator (1008-1, 1008-2) to the absorber (1006-1, 1006-2), wherein the CO 2 or H 2 S reducing system (10A, 10B, 10C, 10D) further comprises a cooling-water supply-line facility (15, 1021-1, 1021-2) that supplies a cooling water to cool the lean solutions (1009-1, 1009-2) of the first and second apparatuses (11-1, 11-2), wherein out of the lean solutions (1009-1, 1009-2) that are supplied from the regenerator (1008-1, 1088-2) to the absorber (1006-1, 1006-2), the lean solution of the first apparatus is arranged at the upstream side of the cooling-water supply-line facility and the lean solution of the second apparatus is arranged at the downstream side of the cooling-water supply-line facility and wherein the absorber of the first CO 2 or H 2 S reducing apparatus has a temperature lower than that of the absorber of the second CO 2 or H 2 S reducing apparatus, the regenerator of the first CO 2 or H 2 S reducing apparatus has a temperature lower than that of the regenerator of the second CO 2 or H 2 S reducing apparatus, the absorbing solutions (1005-1, 1005-2) of the first and second reducing apparatuses (11-1, 11-2), which are different kinds of absorbing solutions, absorb at least one of CO 2 and H 2 S and are regenerated depending on a temperature thereof, and the provided flue gas is fed continuously, in order, to the first reducing apparatus and subsequently to the second apparatus. an absorber (1006-1, 1006-2) that reduces at least one of CO 2 and H 2 S by bringing a provided flue gas including at least one of CO 2 and H 2 S into contact with an absorbing solution (1005-1, 1005-2), a regenerator (1008-1, 1008-2) that regenerates a rich solution (1007-1, 1007-2) that has absorbed at least one of CO 2 and H 2 S, a rich-solution supply line (12-1, 12-2) that feeds the rich solution (1007-1, 1007-2) from the absorber (1006-1, 1006-2) to the regenerator (1008-1, 1008-2), and a lean-solution supply line (13-1, 13-2) that feeds a lean solution (1009-1, 1009-2) from which at least one of CO 2 and H 2 S are reduced in the regenerator (1008-1, 1008-2), from the regenerator (1008-1, 1008-2) to the absorber (1006-1, 1006-2), wherein the CO 2 or H 2 S reducing system (10A, 10B, 10C, 10D) further comprises a cooling-water supply-line facility (15, 1021-1, 1021-2) that supplies a cooling water to cool the lean solutions (1009-1, 1009-2) of the first and second apparatuses (11-1, 11-2), wherein out of the lean solutions (1009-1, 1009-2) that are supplied from the regenerator (1008-1, 1088-2) to the absorber (1006-1, 1006-2), the lean solution of the first apparatus is arranged at the upstream side of the cooling-water supply-line facility and the lean solution of the second apparatus is arranged at the downstream side of the cooling-water supply-line facility and wherein the absorber of the first CO 2 or H 2 S reducing apparatus has a temperature lower than that of the absorber of the second CO 2 or H 2 S reducing apparatus, the regenerator of the first CO 2 or H 2 S reducing apparatus has a temperature lower than that of the regenerator of the second CO 2 or H 2 S reducing apparatus, the absorbing solutions (1005-1, 1005-2) of the first and second reducing apparatuses (11-1, 11-2), which are different kinds of absorbing solutions, absorb at least one of CO 2 and H 2 S and are regenerated depending on a temperature thereof, and the provided flue gas is fed continuously, in order, to the first reducing apparatus and subsequently to the second apparatus.