A 62-year-old woman with rheumatoid arthritis had been treated with prednisolone (PSL) 6 mg/day and methotrexate (MTX) 8 mg/week for 12 years. Because her arthritis was active, abatacept was added to the treatment five months ago. She was referred to our hospital because of dry cough and fever, which continued for two weeks. She had hypoxemia, and her chest computed tomography (CT) revealed bilateral diffuse ground-glass opacities. We made a diagnosis of pneumocytis pneumonia (PCP) by bronchoalveolar lavage, in which Pneumocystis jirovecii was detected by Grocott staining. After administration of co-trimoxazole (sulfamethoxazol-trimethoprim) and steroid pulse therapy, her clinical condition improved dramatically. Abatacept was released as one of the new biological drugs for rheumatoid arthritis on September 2010, and there has yet been no report of PCP cases with it. In the U.S. and Europe, it has been reported that severe infection is complicated infrequently during treatment with abatacept, but eight cases of PCP have been found in postmarketing surveillance in Japan. We must use abatacept carefully, as with other biological drugs, for the treatment of RA.