A 14-year-old girl underwent a medical checkup for Mycobacterium tuberculosis infection because her grandmother had been diagnosed with pulmonary tuberculosis three months earlier. The interferon-gamma release assay (IGRA) showed a positive result. The patient's chest X-ray findings were normal. Chest computed tomography (CT) showed a single mass lesion in the right lower lobe of the lung. A sputum smear of acid-fast bacilli was positive; however, the polymerase chain reaction results for tuberculosis were negative. We diagnosed the patient with pulmonary tuberculosis based on the fact that she had come in contact with a tuberculosis patient. Six weeks later, a liquid culture examination for acid-fast bacilli was found to be positive and the acid-fast bacillus was identified as M. tuberculosis. The use of chest CT is not routinely recommended in all children suspected of having M. tuberculosis infection. However, IGRA-positive children who report frequent contact with infected individuals should undergo CT tomography if chest X-rays do not show any abnormal shadows.