To provide a method for producing a high-purity crystalline methylisourea hydrochloride.

(a) A methanol solution or suspension of O-methyliso urea hydrochloride is reacted with an alkali metal acetate at -10 to 65°C, (b) a precipitated alkali metal chloride is removed from the reaction mixture in a temperature range based on the process (a) and (c) the reaction system is cooled to -50 to 30°C to crystallize Omethylisourea acetate, which is separated by an ordinary method. O-Methylisourea acetate can be obtained in a crystal form in an extremely excellent yield and purity.