A six-month-old girl with a 5 consecutive day history of constipation and poor feeding developed generalized weakness, poor head control, difficulties in sucking and swallowing, and cranial nerve dysfunction within a few days. These characteristic manifestations and clinical course prompted examination of the possibility of infant botulism, although no history of eating honey was obtained. Mouse bioassay performed with enema effluent demonstrated type B botulinum toxin. Culture of the effluent was positive for Clostridium botulinum type B. This is the first case of type B infant botulism in Japan.