Because there is no fundamental treatment for Down's syndrome, and participation in society is the ultimate goal, the present invention provides a remedy and method for treating Down's syndrome that improve congenital delayed psychomotor development, and promote improvement in the performance of daily activities and socialization. The present invention is directed to a therapeutic agent for Down's syndrome that contains an acetylcholinesterase inhibitor, a therapeutic agent for improving mental retardation in Down's syndrome, and an agent for improving daily activities of patients. Specifically, the acetylcholinesterase inhibitor is donepezil hydrochloride. More specifically, the present invention is directed to a therapeutic agent for Down's syndrome, a therapeutic agent for improving mental retardation in Down's syndrome, and an agent for improving daily activities of patients with an IQ of 48 or lower and/or patients aged 36 or younger.