Choledochal cyst is a congenital anomaly with classic triad of abdominal pain, jaundice and right upper abdominal mass. Bile peritonitis caused by cyst rupture is relatively not rare in infancy. The mechanism of rupture must be epithelial irritation of the biliary tract by refluxed pancreatic juice caused by pancreatico-biliary malunion associated with mural immaturity in infancy, rather than an abnormal rise in ductal pressure or congenital mural weakness at a certain point. We experienced a case of bile peritonitis caused by spontanenous rupture of choledochal cyst in a 10-month-old girl presented with abdominal distension, persistent fever, diarrhea, irritability and intractable ascites. She was presumed as having bile peritonitis by bile colored ascitic fluid with elevated bilirubin level and diagnosis was made by 99mTc DISIDA hepatobiliary scan showing extrahepatic biliary leak. The perforated cyst was surgically removed and the biliary tree was reconstructed with a Roux-en-Y hepaticojejunostomy.