We reported a case of pulmonary infection caused by Mycobacterium szulgai (M. szulgai) in an immunocompetent, asymptomatic 55-year-old man without underlying disease. A chest radiograph of an annual health examination revealed a right upper lobe infiltrate with thin-walled cavities, which was not present in the previous year. An acid-fast stain of bronchial washing fluid was positive, and antimycobacterial chemotherapy with isoniazid (400 mg/day), rifampin (450 mg/day), and ethambutol (750 mg/day) was initiated on presumptive diagnosis of the case as tuberculosis. DNA-DNA hybridization of sputum and bronchial washing samples identified M. szulgai as the causative organism. Antimicrobial susceptibility testing indicated that the isolate was sensitive to most common antimycobacterial drugs except capreomycin (CPM) and p-aminosalicylic acid (PAS), and was also sensitive to clarithromycin and fluoroquinolones including ofloxacin, levofloxacin, sparfloxacin, and ciprofloxacin. After 12 months of therapy, a repeat chest radiograph demonstrated improvement of the right upper lobe infiltrate. When M. szulgai is isolated, it almost always represents a true pathogen. Therefore, the detection of even a small number of M. szulgai warrants treatment based on susceptibility testing.