The patient was a 5 years old male who had had the cyanosis and congestive heart failure from his neonatal period. Dopamine, digitalis and diuretics disappeared his symptoms and he had been followed up as the out-patient. Preoperative cardiac catheterization revealed atrial septal defect and moderately pulmonary stenosis with two-staged systolic pressure gradient in a right ventricular cavity. Right ventriculogram showed subpulmonary crescent-shaped, linear filling defect. Ventricular septal defect was not detected. He was underwent open heart surgery and subpulmonary membranous stenosis was found out. Pressure gradient across the right ventricular outflow tract was diminished by the resection of the membranous structure. Atrial septal defect without lower margin was closed directly. Postoperative course was uneventful. Right ventricular apical systolic pressure was decreased to the degree of 27 mmHg postoperatively.