A 70-year-old man who presented with fever was diagnosed and treated as bacterial pneumonia in a previous hospital. Despite treatments with several antibiotics, fever and shortness of breath had sustained for almost one month. His hypoxia progressed, and his radiographic findings deteriorated with ground-glass opacities and consolidations; therefore he was transferred to our hospital. We performed bronchoscopy, and Pneumocystis jirovecii was detected by Grocott staining of bronchoalveolar lavage fluid. We then made a diagnosis of Pneumocystis pneumonia (PCP). Furthermore, he was diagnosed as adult T-cell leukemia (ATL) with lymph node biopsy. His clinical condition and radiographic images gradually improved after administration of sulfamethoxazole-trimethoprim and steroid, but cystic formation, relatively common among PCP in a patient with human immunodeficiency virus (HIV), was developed. We reported this case as PCP in a patient with ATL presenting cystic formation usually observed in HIV-PCP.