A 79-year-old woman attended our hospital because of reticular opacification on chest computed tomography scan, and she described having had dry cough for a year. We considered the abnormal scans to be indeterminate for usual interstitial pneumonia pattern. Her serum autoantibodies were almost all negative, except for a rheumatoid factor of 114 U/mL. Five months later we decided to refer her to another hospital where cryobiopsy was available. Blood tests were repeated and were now positive for anti-citrullinated peptide antibodies (ACPA). The bronchoscopy revealed no specific findings in her lung tissues. Pirfenidone therapy was begun at the hospital. She came back to our hospital after a further three months and we continued the therapy without serious problems. A year since her first visit to our hospital she complained of joint pain in her fingers and wrists. Her 2010 ACR/EULAR rheumatoid arthritis classification criteria score was 7. After consultation with a rheumatologist, she was diagnosed with rheumatoid arthritis. Although the connection between positive ACPA and the development of rheumatoid arthritis has been recognized, no report discussing its positive conversion among interstitial pneumoniae cases has been made up until now.