A method of manufacturing a copolymer, the method comprising copolymerizing a conjugated diene compound and an olefin consisting of an α-olefin in the presence of a polymerization catalyst composition, the polymerization catalyst composition containing: component: a rare earth element compound or a reactant of the rare earth element compound and a Lewis base, with no bond formed between the rare earth element and carbon; and component: an ionic compound (B-1) composed of a non-coordinating anion and a cation, and, the polymerization catalyst composition further containing: component: an organic metal compound represented by the following general formula (i): YR 1 a R 2 b R 3 c (i) where Y is a metal selected from Group 1, Group 2, Group 12, and Group 13 of the periodic table; R 1 and R 2 are the same or different hydrocarbon groups each having 1 to 10 carbon atoms or a hydrogen atom; and R 3 is a hydrocarbon group having 1 to 10 carbon atoms, in which R 3 may be the same as or different from R 1 or R 2 above, with a being 1 and b and c both being 0 when Y is a metal selected from Group 1 of the periodic table, a and b being 1 and c being 0 when Y is a metal selected from Group 2 and Group 12 of the periodic table, a, b, and c are all 1 when Y is a metal selected from Group 13 of the periodic table, wherein as the component, a rare earth element compound or a reactant of the rare earth element compound and a Lewis base is represented by the following general formula or: MX 2 ·Lw MX 3 ·Lw, where M represents gadolinium; X each independently represents a hydrogen atom, a halogen atom, an alkoxy group, a thiolate group, an amide group, a silyl group, an aldehyde residue, a ketone residue, a carboxylic acid residue, a thiocarboxylic acid residue, or a phosphorous compound residue; L represents a Lewis base; and w represents 0 to 3. component: a rare earth element compound or a reactant of the rare earth element compound and a Lewis base, with no bond formed between the rare earth element and carbon; and component: an ionic compound (B-1) composed of a non-coordinating anion and a cation, and, the polymerization catalyst composition further containing: component: an organic metal compound represented by the following general formula (i): YR 1 a R 2 b R 3 c (i) where Y is a metal selected from Group 1, Group 2, Group 12, and Group 13 of the periodic table; R 1 and R 2 are the same or different hydrocarbon groups each having 1 to 10 carbon atoms or a hydrogen atom; and R 3 is a hydrocarbon group having 1 to 10 carbon atoms, in which R 3 may be the same as or different from R 1 or R 2 above, with a being 1 and b and c both being 0 when Y is a metal selected from Group 1 of the periodic table, a and b being 1 and c being 0 when Y is a metal selected from Group 2 and Group 12 of the periodic table, a, b, and c are all 1 when Y is a metal selected from Group 13 of the periodic table, wherein as the component, a rare earth element compound or a reactant of the rare earth element compound and a Lewis base is represented by the following general formula or: MX 2 ·Lw MX 3 ·Lw, where M represents gadolinium; X each independently represents a hydrogen atom, a halogen atom, an alkoxy group, a thiolate group, an amide group, a silyl group, an aldehyde residue, a ketone residue, a carboxylic acid residue, a thiocarboxylic acid residue, or a phosphorous compound residue; L represents a Lewis base; and w represents 0 to 3.