To use a specific alkyldicarboxylic acid to reduce the water absorption of a specific dienic copolymer, thus producing the dienic copolymer whose transparency and haze-free state are maintained even in a highly moist environment.

A 9 to 16C alkyldicarboxylic acid is used for neutralizing the alkali metal derivative of a vinyl aromatic compound-conjugated diene copolymer, thus reducing the water absorption of the copolymer. The method for producing the vinyl aromatic compound-conjugated diene copolymer preferably comprises copolymerizing at least one vinyl aromatic compound with at least one conjugated diene in the presence of a catalyst (for example, n-butyl lithium) based on an alkali metal in a solvent to produce the copolymer chain, and then treating the copolymer with a 9 to 16C alkyldicarboxylic acid (for example, azelaic acid) in the solution.