Tokyo: Japan`s Emperor Akihito was diagnosed with arteriosclerosis -- hardening of the arteries -- following hospital tests on Friday, a report said.
The 77-year-old monarch is expected to continue carrying out his normal duties but will need to take medication, Kyodo News agency said, quoting the imperial household agency.
Doctors ordered tests after a routine check-up in January revealed symptoms of insufficient blood flow to the heart.
Akihito was accompanied to the hospital in central Tokyo by Empress Michiko, 76, and is expected to spend the night there before returning to the Imperial Palace on Saturday.
The emperor, who acceded to the throne in 1989 following the death of his father Emperor Hirohito, underwent surgery for prostate cancer in 2003.
As the titular head of state in Japan, he plays a largely ceremonial role but is held in deep respect by many Japanese.