A method for producing a thermoplastic elastomer, the thermoplastic elastomer including: a block (a) containing a C4-C7 isoolefin monomer (A) and alkylstyrene (B) as main components; and a block (b) containing an aromatic vinyl monomer (C) as a main component, the alkylstyrene (B) being represented by the following general formula : wherein R 1 and R 2 are each selected from the group consisting of a hydrogen atom, a halogen atom, a C1-C5 alkyl group, and a C1-C5 halogenated alkyl group; and R 1 and R 2 may be identical or different, said method comprising the steps of: using a compound as a polymerization initiator; and maintaining an amount of substance of an unreacted portion of the alkylstyrene (B) during polymerization of the block (a) at a molar ratio of not more than 1/90 relative to a total amount of substance of the isoolefin monomer (A), the compound being represented by the following general formula : wherein R 3 s may be identical or different, and each represent a hydrogen atom or a C1-C6 monovalent hydrocarbon group; R 4 represents a monovalent or polyvalent aromatic hydrocarbon group, or a monovalent or polyvalent aliphatic hydrocarbon group; X represents a halogen atom, a C1-C6 alkoxyl group, or a C1-C6 acyloxy group; n represents an integer of 1 to 6; and X may be identical or different where X is plural. a block (a) containing a C4-C7 isoolefin monomer (A) and alkylstyrene (B) as main components; and a block (b) containing an aromatic vinyl monomer (C) as a main component, the alkylstyrene (B) being represented by the following general formula : wherein R 1 and R 2 are each selected from the group consisting of a hydrogen atom, a halogen atom, a C1-C5 alkyl group, and a C1-C5 halogenated alkyl group; and R 1 and R 2 may be identical or different, said method comprising the steps of: using a compound as a polymerization initiator; and maintaining an amount of substance of an unreacted portion of the alkylstyrene (B) during polymerization of the block (a) at a molar ratio of not more than 1/90 relative to a total amount of substance of the isoolefin monomer (A), the compound being represented by the following general formula : wherein R 3 s may be identical or different, and each represent a hydrogen atom or a C1-C6 monovalent hydrocarbon group; R 4 represents a monovalent or polyvalent aromatic hydrocarbon group, or a monovalent or polyvalent aliphatic hydrocarbon group; X represents a halogen atom, a C1-C6 alkoxyl group, or a C1-C6 acyloxy group; n represents an integer of 1 to 6; and X may be identical or different where X is plural. using a compound as a polymerization initiator; and maintaining an amount of substance of an unreacted portion of the alkylstyrene (B) during polymerization of the block (a) at a molar ratio of not more than 1/90 relative to a total amount of substance of the isoolefin monomer (A), the compound being represented by the following general formula : wherein R 3 s may be identical or different, and each represent a hydrogen atom or a C1-C6 monovalent hydrocarbon group; R 4 represents a monovalent or polyvalent aromatic hydrocarbon group, or a monovalent or polyvalent aliphatic hydrocarbon group; X represents a halogen atom, a C1-C6 alkoxyl group, or a C1-C6 acyloxy group; n represents an integer of 1 to 6; and X may be identical or different where X is plural. <EOS>