The case is 47-year-old, homeless man. He was diagnosed as cavitary, sputum smear positive pan-sensitive lung tuberculosis, and admitted to TB ward of our hospital. At the age of 4-year-old he had tuberculous hilar lymphadenopathy and took medicine. He had no other associated disease, and HIV test was negative. He started standard chemotherapy and 2 months later his sputum culture was converted to negative. His adherence to medicine was thought to be good. But about 2 weeks after the sputum conversion, his sputum culture was re-converted to positive for Mycobacterium tuberculosis. Thereafter, during and after the completion of standard chemotherapy, his sputum culture had been intermittently positive. The drug sensitivity tests of the strain after re-conversion showed multi-drug resistance. RFLP analysis revealed that the strain before conversion was totally different strain from the strain after re-conversion to positive. The case was considered to be caused by the double infection of MDR strain of Mycobacterium tuberculosis during the course of treatment for tuberculosis due to a sensitive strain of Mycobacterium tuberculosis.