The Emperor was admitted to the University of Tokyo Hospital in Bunkyo Ward, Tokyo, on Friday morning ahead of a coronary artery bypass operation scheduled for Saturday.
The Emperor has been diagnosed with angina, which causes a shortage of blood in heart muscle during physical activity.
The operation, which will start on Saturday morning, is expected to be completed after about 3-1/2 hours. The Emperor is expected to leave the operating room on Saturday afternoon if the surgery goes smoothly, the Imperial Household Agency said.
In the event that there are no complications, the Emperor will be released from the hospital in about two weeks at the earliest.
The Emperor and Empress left the Imperial Palace at 10:05 a.m. (09:05 am local time) by car on Friday.
They arrived at the entrance of the hospital at 10:14 a.m., entering a building for inpatients after being received by Takashi Kadowaki, the hospital's director, and Ryozo Nagai, a professor of cardiology at the hospital.
At 10 a.m. the Imperial Palace started allowing people who hope for a speedy recovery of the Emperor to express their best wishes in visitor's books. There was a line of people praying for the operation's success.