A 55-year-old non-smoking woman was admitted to our hospital for re-evaluation of unimproved peripheral ground-glass opacities despite prednisolone and cyclosporine treatment. She was diagnosed with autoimmune pulmonary alveolar proteinosis (PAP) based on transbronchial lung biopsy and granulocyte/macrophage colony-stimulating factor (GM-CSF) antibody testing. GM-CSF inhalation therapy markedly improved the opacities. Bilateral, centrally located lung opacities are typical in PAP, however 10 PAP cases with peripheral infiltration were reported in Japan recently, of which GM-CSF antibody was positive in six. To avoid inappropriate immunosuppressant treatment, PAP should be considered in the differential diagnosis of such peripheral opacities. GM-CSF antibody might be useful for diagnosis.