Emperor Akihito went into hospital Friday, a day before he is due to have heart bypass surgery, imperial palace officials said, as members of the public signed books at the Imperial Palace to wish him well.
The 78-year-old monarch arrived at the University of Tokyo Hospital shortly after 10 a.m., 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 last 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.
During his operation and recuperation, Akihito's first son, Crown Prince Naruhito, will take over his duties such as attending public ceremonies and meeting state guests.
Prime Minister Yoshihiko Noda was among more than 300 people who visited the Imperial Palace to enter their names on books prepared for goodwill wishes.
"I hope he will recuperate as soon as possible," a man who visited the palace said on television footage.
On Thursday, the emperor met 107 elementary school principals at the palace and, referring to the March 11 earthquake-tsunami told them, "I hope you will make efforts in disaster prevention education without forgetting the disaster."
He then had dinner with Empress Michiko, his second son Prince Akishino and his wife Princess Kiko, local reports said.
The surgery comes amid increasing concerns about Akihito's health.
A catheter angiogram on Saturday showed that his arteries continued to narrow since an examination he underwent a year ago, the palace said in a statement.
Doctors have reached "a conclusion that a new action has to be taken" to stop the trend, it said.
In November Akihito, who ascended the throne in 1989 following the death of his father Hirohito, spent 19 days in hospital with mild pneumonia while he underwent surgery for prostate cancer in 2003.
His youngest son Prince Akishino has suggested there should be a discussion about setting a retirement age for the head of state.
Despite being stripped of much of its mystique and its quasi-divine status in the aftermath of World War II, the Japanese throne is held in deep respect by much of the public.© AFP