We report a 67-year-old woman who developed Pneumocystis jirovecii pneumonia (PJP) as a complication of treatment with immunosuppressant, such as methotrexate and leflunomide, for rheumatoid arthritis. She had mild fever and non-productive cough, but there was neither symptoms of dyspnea nor typical X-ray findings at initial presentation. However, rapid progression on a chest X-ray, respiratory symptoms and findings of high-resolution chest computed tomography in a few days suggested a possibility of PJP infection. A polymerase chain reaction-based detection of Pneumocystis jirovecii in induced sputum allowed us an early diagnosis of PJP. Through a preemptive treatment with Trimethoprim-sulfamethoxazole, she fully recovered from PJP.