Emperor Akihito entered the University of Tokyo Hospital on Friday morning to prepare for heart bypass surgery.
The coronary-artery bypass will be conducted Saturday morning by a team of doctors from the hospital and Juntendo University Hospital, and is likely to take about five hours, according to the Imperial Household Agency.
The doctors will explain the surgery's results at a news conference afterward.
The Emperor is expected to be able to leave the hospital in about two weeks if there are no complications.
The 78-year-old Emperor was diagnosed with angina after an angiogram performed Feb. 11 at the hospital showed the narrowing of two of his heart's three coronary arteries, including one that had narrowed further from a year earlier.
For the surgery, the doctors plan to use a blood vessel from another part of the body to create a bypass without stopping the Emperor's heart, the agency said.
The Emperor performed his official duties until Thursday.