The number of patients with measles sharply increased over the first 3 1/2 months of this year, the Health, Labor and Welfare Ministry said.
The ministry and the National Institute of Infectious Diseases (NIID) are warning the public to be on the alert against the epidemic as the Golden Week holiday period gets underway.
The NIID confirmed that there were 139 patients with measles between January and April 16 this year, compared with 159 throughout last year. Those in their 20s and 30s each accounted for 30 percent of the total.
Yamagata Prefecture had the largest number of patients, at 50, followed by Mie (20), Tokyo (16) and Hiroshima (11). Twenty-one patients are believed to have been infected with the highly contagious disease overseas, mostly in Southeast Asian countries including Indonesia.
Patients are said to show such symptoms as a high fever, cough and runny nose after about 10 days after being infected, and subsequently develop a rash.
Last year, a group infection was confirmed at Kansai International Airport in Osaka Prefecture.
A man living in Yamagata Prefecture showed symptoms of measles in March after coming back from Indonesia, and the disease subsequently spread throughout the prefecture.
An official of the Health, Labor and Welfare Ministry urged the public to consult a doctor if they came down with any symptoms of measles.
"The initial symptoms resemble those of cold. If patients are late in undergoing treatment, the disease could spread further," the official said.
The ministry also recommends those who travel overseas get vaccinated against measles in advance.