Polymerization catalyst composition for polyisoprene synthesis containing: a rare earth element compound (component) represented by the following general formula (i): M-(NQ 1)(NQ 2)(NQ 3) (i) provided that M is gadolinium; and NQ 1, NQ 2, and NQ 3 are of amide group and may be either the same or different from one another and have M-N binding; at least one (component) selected from an ionic compound and a halogen compound; a compound (component) represented by the following general formula: YR 1 a R 2 b R 3 c provided that Y is metal selected from Group 1, Group 2, Group 12, and Group 13 of the periodic table; R 1 and R 2 are of a hydrocarbon group having 1 to 10 carbon atoms or a hydrogen atom; R 3 is of the hydrocarbon group having 1 to 10 carbon atoms; R 1, R 2, and R 3 may be the same or different from one another; when Y is metal selected from Group 1 of the periodic table, a is 1 and b and c are 0; when Y is metal selected from Group 2 or Group 12 of the periodic table, a and b are 1 and c is 0; and when Y is metal selected from Group 13 of the periodic table, a, b, and c are all 1; and isoprene monomer, wherein: a combining molar ratio of the isoprene monomer, the component, and the component satisfies: (isoprene monomer)/(component) is at least 5000, (component /(component) is at least 10, and (isoprene monomer)/(component) is no more than 5000. a rare earth element compound (component) represented by the following general formula (i): M-(NQ 1)(NQ 2)(NQ 3) (i) provided that M is gadolinium; and NQ 1, NQ 2, and NQ 3 are of amide group and may be either the same or different from one another and have M-N binding; at least one (component) selected from an ionic compound and a halogen compound; a compound (component) represented by the following general formula: YR 1 a R 2 b R 3 c provided that Y is metal selected from Group 1, Group 2, Group 12, and Group 13 of the periodic table; R 1 and R 2 are of a hydrocarbon group having 1 to 10 carbon atoms or a hydrogen atom; R 3 is of the hydrocarbon group having 1 to 10 carbon atoms; R 1, R 2, and R 3 may be the same or different from one another; when Y is metal selected from Group 1 of the periodic table, a is 1 and b and c are 0; when Y is metal selected from Group 2 or Group 12 of the periodic table, a and b are 1 and c is 0; and when Y is metal selected from Group 13 of the periodic table, a, b, and c are all 1; and isoprene monomer, wherein: a combining molar ratio of the isoprene monomer, the component, and the component satisfies: (isoprene monomer)/(component) is at least 5000, (component /(component) is at least 10, and (isoprene monomer)/(component) is no more than 5000. provided that M is gadolinium; and NQ 1, NQ 2, and NQ 3 are of amide group and may be either the same or different from one another and have M-N binding; at least one (component) selected from an ionic compound and a halogen compound; a compound (component) represented by the following general formula: YR 1 a R 2 b R 3 c provided that Y is metal selected from Group 1, Group 2, Group 12, and Group 13 of the periodic table; R 1 and R 2 are of a hydrocarbon group having 1 to 10 carbon atoms or a hydrogen atom; R 3 is of the hydrocarbon group having 1 to 10 carbon atoms; R 1, R 2, and R 3 may be the same or different from one another; when Y is metal selected from Group 1 of the periodic table, a is 1 and b and c are 0; when Y is metal selected from Group 2 or Group 12 of the periodic table, a and b are 1 and c is 0; and when Y is metal selected from Group 13 of the periodic table, a, b, and c are all 1; and isoprene monomer, a combining molar ratio of the isoprene monomer, the component, and the component satisfies: (isoprene monomer)/(component) is at least 5000, (component /(component) is at least 10, and (isoprene monomer)/(component) is no more than 5000.