A 73-year-old man was initially diagnosed with idiopathic interstitial pneumonia (IIP) based on his clinical manifestations. A computed tomography (CT) scan of the chest revealed reticular shadows with traction bronchiectasis but no ground-glass opacification. The patient was treated with corticosteroids and immunosuppressants. The patient’s dyspnea progressed gradually, and after 20 months of treatment, ground-glass opacification was revealed on chest CT scan. Bronchoalveolar lavage was performed for diagnostic purposes. Foamy alveolar macrophages and accumulated material stained with periodic acid-Schiff (PAS) were observed in the bronchoalveolar lavage fluid (BALF) and elevated anti-granulocyte-macrophage colony-stimulating factor (GM-CSF) autoantibodies were detected in the serum. Finally, the patient was diagnosed with autoimmune pulmonary alveolar proteinosis (aPAP). BALF specimens from the initial examination retrospectively revealed similar material stained with PAS, and the concentration of anti-GM-CSF autoantibodies in the preserved serum was also elevated. Therefore, it was concluded that the IIP patient had initially experienced subclinical aPAP without the typical clinical and radiologic manifestations.