Japanese Emperor Akihito should cut down his duties following his heart bypass last weekend, his son and heir Crown Prince Naruhito said in comments released to mark his 52nd birthday on Thursday.

"Considering his age, I think it is necessary to reduce his burden," he told reporters, in his first comments on his father's future role.

"It's important that people surrounding him think hard to help him. I'd like to do whatever I can that is helpful in this regard," he said at the news conference held on Tuesday but embargoed until Thursday.

The 78-year-old emperor left intensive care on Monday, just two days after undergoing a successful heart bypass operation.

During his recuperation, Naruhito will handle official duties such as attending public ceremonies and meeting state guests.

Referring to his experience in November in acting for his father when the emperor was hospitalised for bronchial pneumonia, Naruhito said: "I keenly felt the need to study more (about the emperor's duty) to help His Imperial Majesty."

The surgery highlighted concerns about Akihito's health and has also raised questions about the rules of succession in the monarchy, which allows only males to become heirs, while women are forced to leave the household if they marry a commoner.

Naruhito's comments are the first in reference to the increasing ill-health of his father, who also underwent surgery for prostate cancer in 2003, and still receives treatment.

However, last year his younger brother Prince Akishino said the country should look to setting a retirement age for the emperor, as Akihito recovered from pneumonia.

"I think it will become necessary," he said, when asked by a reporter to comment on the idea.

"When you pass a certain age, it gradually becomes difficult for people to do various things. I think it is an idea" to set a retirement age, Akishino said at the time.

Japan's royals rarely comment on public or political matters, including those touching on the affairs of their own cloistered family.

Akishino's five-year-old son Prince Hisahito is the only grandson of the ageing emperor.

The Japanese government plans to study a reform to the Imperial House Law so that women can remain as royal family members after marriage.