A 37-year-old male was admitted to our hospital because of abnormal findings seen in a chest radiograph from a physical examination. There was nothing unusual about his past history, and he had been healthy. Chest radiography showed an infiltrative shadow with a cavity in the right upper lobe. A smear test of the sputum was negative but a culture was positive for a mycobacterium, which was identified as M. szulgai. A diagnosis of atypical pulmonary mycobacteriosis caused by M. szulgai. The patient was treated with isoniazid, rifampicin and ethambutol, which gradually improved the infiltration shadow, and his sputum cultures became negative for mycobacteria. In recent years, the frequency of atypical mycobacteriosis has increased up to 10% to 15% of all mycobacterial infections. In Japan, M. avium complex and M. kansasii account for more than 90% of such infections, so other isolates are rare. Most previous cases of atypical mycobacteriosis were secondary infections associated with underlying disease. In this patient, however, we describe a rare case of pulmonary infection due to M. szulgai appearing in a healthy male without underlying disease.