Budd-Chiari syndrome is a rather unusual clinical situation caused by occlusion of the hepatic vein of inferior vena cava, the classical triad of which are abdominal pain, ascites and hepatomegaly. A 29-year-old gravida 3 para 1 woman delivered an immature male baby weighing 2172 g with an Apgar score of 9 points at 35 weeks and 3 days of gestation. She was transferred to the National Hospital Organization Osaka National Hospital 112 days after delivery due to the sudden development of massive ascites. Magnetic resonance angiography and enhanced computed tomography detected the occlusion by thrombosis of both the middle and left hepatic veins, so she was diagnosed with Budd-Chiari syndrome. Her protein C antigen and activity were 37% and 50%, respectively, corresponding to type 1 protein C deficiency. Conservative treatment by continuous oral treatment of spironolactone (25 mg/day), furosemide (20 mg/day) and prophylactic warfarin (2 mg/day) much improved the ascites.