The incidence of measles epidemics has decreased recently owing to the development and widespread use of measles vaccine in the United States of America and Europe. However, repeated measles epidemics have been reported in Japan. Here, the authors report a case of an extremely low body weight infant (ELBWI) whose mother had a measles virus infection. Real-time PCR was performed on the infant's blood and urine samples and skin, nasal secretion, and anal swabs, as well as on the mother's breast milk, blood samples and throat swabs. The infant was found to be positive for measles virus by PCR, but not immunoglobulin M positive. An earlier report showed that there were no such cases in which the patient was found to be positive for measles virus by real-time PCR but was not infected by the measles virus.