A 19-year-old man had a fever of more than 39 degrees Celsius (102 degrees Fahrenheit) and a rash over his body after returning from Indonesia earlier this month, the Disease Control and Prevention Center (DCC) and local media have said.
Despite the condition, the man, whose name was withheld, went to Bieber's concert on August 14 at Makuhari Messe in Chiba, east of Tokyo, where some 25,000 fans gathered for the event, they said.
The man, from the western Japanese city of Nishinomiya, then stopped in Tokyo and neighbouring Kanagawa prefecture before returning home on August 19, when he was finally diagnosed with measles.
"Measles is highly infectious," an official at Japan's health and welfare ministry told AFP.
"It could lead to a fatal case," he added, issuing warnings to hospitals across the nation to be aware of an outbreak.
Concert organisers on their website also asked those who attended to see doctors if they developed symptoms.
So far, however, there have been no reports of anyone who attended the concert contracting measles.