We report a 51-year-old male patient with tetralogy of Fallot who had an anoxic spell during pre-bypass period before intracardiac repair. Immediately after starting sternotomy, SpO2 decreased suddenly from 83% to 30% with PaO2 of 26 mmHg. Heart rate showed an increase to 120-140 bpm with little change in blood pressure. We considered that the causes of this spell were 1) light level of anesthesia, 2) hypoxemia and 3) lack of intravascular blood volume. This case warns us that we should take care to avoid an anoxic spell in an adult case for intracardiac repair.