Kaposi's sarcoma (KS) is one of the diseases related to acquired immunodeficiency syndrome (AIDS) and the most common malignant tumor in patients with human immunodeficiency virus (HIV) infection. The high-resolution computed tomography (HRCT) may show characteristic images and may help us make a diagnosis of lung KS. We report a 33-year-old man with AIDS showing diffuse ground-glass opacities (GGO) in chest computed tomography (CT). We performed a series of tests, including HIV testing (Western blot method and viral RNA), β-D glucan, Pneumocystis jirovecii PCR of the intratracheal aspiration, and HRCT. They showed diffuse GGO accompanied by intralobular septal thickness in bilateral lung field. Accordingly, he was diagnosed as having AIDS-related P. jirovecii pneumonia. Despite prompt initiation of the treatment, the patient succumbed to pneumonia 26 days after hospitalization. The pathological examination in an autopsy revealed multiple nodular lesions of variable size in both lungs, which were not detectable by HRCT. A proliferation of elongated spindle-shaped cells embedding abnormal vascular slits was observed, and we made a diagnosis of the primary lung KS. This is the first report of primary lung KS without preceding lesions in other organs and showing GGO on HRCT. This case emphasizes the need to include lung KS in differential diagnosis when patients with AIDS present GGO in chest CT.