A novel therapeutic and prophylactic agent for inflammatory bowel diseases and a method for treating inflammatory bowel diseases. The agent comprises a 2-amino-1,3-propanediol derivative (e.g., 2-amino-2-[4-(3-benzyloxyphenylthio)-2-chlorophenyl]ethyl-1,3-propanediol hydrochloride) represented by the chemical formula (1) or a pharmaceutically acceptable salt or hydrate thereof: ##STR00001## The agent is useful in the treatment or prevention of Crohn's disease, Crohn's disease in large intestine, intestinal Behcet's disease, ulcerative colitis, bleeding rectal ulcer and pouchitis.