A method of producing a triblock copolymer, the method including reacting a diblock copolymer that contains a first block formed from a repeating structure including a constituent unit containing an aromatic hydrocarbon group, and a second block formed from a repeating structure including a constituent unit derived from an α-substituted acrylic acid ester with a homopolymer that contains a third block formed from a repeating structure including a constituent unit containing an aromatic hydrocarbon group and containing a hydroxy group bonded to a terminal of a main chain in the third block to obtain a triblock copolymer, in which the reaction is transesterification of the constituent unit derived from an (α-substituted) acrylic acid ester at a terminal of a main chain of the diblock copolymer with the homopolymer.