We present a case of a previously health middleage male patient, without personal history of other condition, who was admitted in our hospital presenting fever, weight loss, and signs and symptoms of acute respiratory distress. The chest computed tomography showed numerous cystic lesions, diffuse ground-glass opacities, honeycombing, and consolidation areas. An HIV infection was confirmed, and the diagnosis of Pneumocystis jirovecii pneumonia was made on induced sputum smear stain. After the initiation of oral treatment with trimethoprim-sulfamethoxazole, the clinical course was rapidly improved. It is important to consider that opportunistic infections such as Pneumocystis jirovecii pneumonia can occur not only in patients previously diagnosed with HIV-infection, but also in patients without a medical history of immunosuppressing disorders.