A 70-year-old man with rheumatoid arthritis (RA) visited our hospital owing to an abnormal shadow observed on his chest computed tomography (CT) scan. As he had received salazosulfapyridine (SASP) for RA, we regarded the irregular shadows on both lungs as part of a drug-induced lung injury. Because the withdrawal of SASP resulted in the exacerbation of RA, his medication was restarted. After 17 months, his lung lesions worsened. The histological findings of a transbronchial lung biopsy revealed that the alveolar spaces were filled with surfactant protein A (SP-A)-positive material. The patient also tested positive for anti-granulocyte-macrophage colony-stimulating factor (GM-CSF) autoantibody in the serum (40.5μg/mL). Based on these findings, the patient was diagnosed with autoimmune pulmonary alveolar proteinosis (aPAP). Because GM-CSF influences the development and pathogenesis of RA, anti GM-CSF autoantibodies will ameliorate RA; however, it can also be a factor in aPAP onset. The abnormal shadow observed on the chest CT scans for the patient improved after his treatment with ambroxol and continued SASP administration.