A 53-year-old man was adimitted to our hospital because of AIDS-related Pneumocystis jirovecii pneumonia. Although the diffuse interstitial infiltrate disappeared on chest radiography after initiation of therapy for P. jirovecii pneumonia, a solitary nodule in the right upper lung field persisted. After initiation of highly active antiretroviral therapy (HAART) (stavudine, lamivudine and nevirapine), the pulmonary nodule became larger and a new lesion in the left upper lung field developed. We diagnosed him as pulmonary Mycobacterium avium infecton in the histological findings of transbronchial lung biopsy (TBLB), and the culture of brushing specimens and gastric juice. Our case has probably been diagnosed as immune reconstuitution syndrome because his pulmonary nodule became larger followed by a decrease of his viral load and an increasse of his CD4+ cell count after commencing HAART.