A 55-year-old man with diabetes mellitus was admitted to our hospital because of abnormal shadows in his chest radiographs. Both chest radiography and CT revealed infiltrative shadows in the right upper lung field. Repeated sputum smears showed no mycobacterium, so bronchoalveolar lavage (BAL) was performed bronchoscopically at the right B3b. The BAL fluid and the sputum obtained on the day after BAL contained acid-fast, branching filamentous structures. The microorganism was identified as Nocardia asteroides. Trimethoprim-sulfamethoxazole (ST) and SPFX were therefore administered. Later, Mycobacterium tuberculosis was detected in a 6-week culture of the sputum and BAL fluid. This case was diagnosed as a mixed infection by Nocardia asteroides and Mycobacterium tuberculosis, so the three anti-tuberculosis agents INH, RFP and EB were added. After 6 months of the combined therapy, neither microorganism could be detected in the sputum, and the lesion in the CT scan had decreased markedly in size. Since such a mixed infection is very rare, no treatment strategy has yet been established. The combined therapy was judged to have been effective in this case.