A 54-year-old man whose awake percutaneous arterial oxygen saturation (SpO2) was 94% was diagnosed with obstructive sleep apnea by polysomnography (PSG). His apnea-hypopnea index (AHI) was 138.8 (AI: 4.7 and HI: 134.1), so he was treated with continuous positive airway pressure (CPAP), and his condition was considered well-controlled by the CPAP tracking system (AHI=3.4), with improvement seen in his symptoms when he left our hospital. However, he returned to our hospital 4 years later with recurrent sleepiness and hypercapnia despite the well-controlled status (AHI=3.8) according to the tracking system. His hypercapnia improved following voluntary hyperventilation. Idiopathic central alveolar hypoventilation was diagnosed, with the AHI considered to be well-controlled by the CPAP tracking system but not at all according to PSG.