A 76-year-old female, who had been under observation for interstitial pneumonia due to rheumatoid arthritis for over 10 years, was admitted to our hospital because of dyspnea. Her chest CT showed consolidation in both lower lung fields positive for FDG-PET. Transbronchial lung biopsy was performed and diffuse large B-cell lymphomas were histologically observed. The patient was treated with 6 cycles of Rituximab-CHOP (cyclophosphamide, doxorubicin, vincristine, prednisolone) and has remained disease free for over three years. This is the first case of primary pulmonary lymphoma developing during the clinical course of interstitial pneumonia associated with rheumatoid arthritis in patients after cessation of methotrexate or with Sjogren syndrome.