Japan's Emperor Akihito, 78, will undergo heart bypass surgery this week after tests showed the narrowing of his arteries has worsened, the palace announced on Sunday.

The operation, scheduled for Saturday, will be performed "to maintain and to improve his majesty's daily life. We have decided to ask his majesty to have a coronary artery bypass surgery," a palace spokesman said.

The announcement came shortly after Akihito was discharged from the University of Tokyo Hospital, where he underwent a catheter angiogram on Saturday.

The test showed that the narrowing of his arteries has progressed, compared with an examination he underwent a year ago, the spokesman said.

Doctors have reached "a conclusion that a new action has to be taken" to stop the trend, he said.

The operation will be performed by physicians from the University of Tokyo and private Juntendo University, the spokesman said.

Akihito, wearing a gray suit and a striped tie, walked out of the university hospital Sunday, accompanied by Empress Michiko, as his physicians stood in line at the entrance.

The surgery comes amid increasing concerns about Akihito's health.

The latest angiogram was arranged after periodic electrocardiograms showed restricted blood flow to his heart.

In November, the emperor spent 19 days in hospital suffering mild pneumonia.

Akihito, who ascended the throne in 1989 following the death of his father Hirohito, underwent surgery for prostate cancer in 2003.

The emperor's second son Prince Akishino in November suggested there should be a discussion about setting a retirement age for the titular 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.