The present invention provides a simple method of producing polymorphic crystal (III), which has high safety to environment and the bodies of operators; is gentle to environment; and can produce at low costs; and has a high refining effect. It is a method of producing polymorphic crystal (III) of donepezil hydrochloride (chemical name: 1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methylpiperidine•monohydrochloride) represented by the following structural formula (formula (I)), which comprises dissolving donepezil (chemical name: 1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methylpiperidine) in ethanol; and adding hydrochloric acid or hydrogen chloride thereto, followed by stirring.