PROBLEM TO BE SOLVED : To provide medicines which contains lactulose as an active ingredient and in which usage and dosage of lactulose are optimized as an agent for treating constipation optimized to Japanese. SOLUTION : Provided is an agent for treating constipation which contains lactulose as an active ingredient, characterized by orally administering lactulose in the amount of 22.5 to 29.5 g divided into twice a day to a Japanese patient with constipation. SELECTED DRAWING : None COPYRIGHT : (C)2019, JPO&INPIT