An 11-month-old boy who had been diagnosed as having a ventricular septal defect and mitral valve stenosis presented with severe pulmonary hypertension. Echocardiography showed a large perimembranous ventricular septal defect and a complete bridge-type double-orifice mitral valve together with an increased left ventricular inflow pressure gradient and overt mitral valve stenosis. Cardiac catheterization revealed severe pulmonary hypertension, but the nitric oxide tolerance test ruled out pulmonary vascular obstructive disease. The ventricular septal defect was surgically closed because the relative severity of the mitral valve stenosis was unknown. The patient remained under postoperative observation due to improved pulmonary hypertension and mild mitral valve stenosis. During evaluation of the double-orifice mitral valve with intracardiac left-right shunt, it was important in terms of the treatment strategy decision to infer the effective mitral orifice area, and cardiac ultrasonography was useful as the preoperative method.