A 16-year-old male consulted a former doctor because of cough, which was diagnosed as a common cold. After taking medicine, including acetaminophen, he developed mucocutaneous lesions and high fever, which was suspected to be Stevens-Johnson Syndrome (SJS) associated with legal drugs, and he was admitted to our hospital. Although chest X-ray showed no abnormal findings, a decrease of oxygen saturation and a high titer of blood serum antibody against Mycoplasma pneumoniae were found. Moreover, chest CT showed bronchiolitis. M. pneumoniae infection was considered to be a cause of SJS, in addition to the drugs. After steroids and antibiotics were administered, mucocutaneous lesions and bronchiolitis had gradually improved. Because results of a patch test for the suspected drugs were negative, he was diagnosed as SJS associated with M. pneumoniae. Examination for detection of M. pneumoniae infection is important even in SJS without an abnormality of a chest X-ray.