According to the present invention, a method of allowing methyl 2-bromo-3-{4-[2-(5-ethyl-2-pyridyl) ethoxy] phenyl} propionate (hereinafter abbreviated as compound (1) A method for inhibiting dehydrobromination of Compound 1 is disclosed. Compound (1) is a therapeutic agent 5-{4-[2-(5-ethyl-2-pyridyl) ethoxy] benzyl}-3-hydroxybenzoic acid which is a therapeutic agent showing excellent therapeutic effect against type 2 diabetes caused by insufficient insulin action, Thiazolidine-2,4-dione hydrochloride as an intermediate material for production. As the acid, a carboxylic acid having 1 to 3 carbon atoms or hydrobromic acid is preferable, and the compounding amount thereof is preferably 0.01 to 2 mol with respect to 1 mol of the compound 1.
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