The number of tuberculosis outbreak among adult groups has increased. In some of them, it is difficult to identify the route of infection and sometimes many people are involved in the outbreak. We experienced a suspected case of mass outbreak in a small company separated into 2 floors. The first patient, who lived in another city, was diagnosed as pulmonary tuberculosis in April with bilateral extensive cavitary lesions complicated with pleurisy on chest X-ray, and his sputum smear was Gaffky No. 8 and culture (+) (resistant to isoniazid). We conducted tuberculin skin test (TST) for those under-30 year-old. In TST, 4 persons showed strong positive reaction. After consulting with tuberculosis specialists, three were diagnosed as newly infected with tuberculosis and one as hilar lymph node tuberculosis. So we added 25 staffs under 60s for TST and found that another 13 people were suspected to be infected with tuberculosis. From the initial information, the index cast was said to work only in the 2nd floor, so the staffs in 7th floor were excluded from the survey. Through the interview with staffs by a public health nurse and an inspection of the work place with sanitary inspectors, it was concluded to expand the survey to the staffs working in the 7th floor, and another newly infected persons were found. Staffs in a small company do not belong to "danger group", but the importance of investigations by public health officials was suggested in the case of suspected outbreak.