Japan's Emperor Akihito went into hospital on Friday, a day before undergoing heart bypass surgery, the Imperial Palace said.
The 78-year-old monarch arrived at the University of Tokyo Hospital shortly after 10 am (0100 GMT) accompanied by his wife Empress Michiko and bowed to doctors before going inside.
"His Majesty arrived in the hospital for the operation," a spokeswoman for the Imperial Household Agency told AFP.
The operation, which was announced on Sunday after tests showed a narrowing of Akihito's arteries, is expected to take about five hours and will be performed by physicians from the University of Tokyo and private Juntendo University.
The emperor is likely to be discharged in about two weeks if he does well after the operation, Jiji Press reported, although the palace would not confirm this.
The operation will be performed "to maintain and to improve his majesty's daily life", a palace spokesman said last Sunday. During his operation and recuperation, Akihito's first son, Crown Prince Naruhito, will take over his duties such as attending public ceremonies and meeting with state guests.