TOKYO — Stress has forced Japan's emperor to step back from the public eye ahead of his 75th birthday Tuesday, with a palace official hinting that one of the causes may be concerns about who will succeed him.

Emperor Akihito was to appear before well-wishers to mark his birthday, but will take the advice of doctors and lighten his official duties as he tries to recover from an irregular pulse and bleeding from his stomach — symptoms imperial doctors have attributed to "mental stress."

Akihito chose to sit out a news conference he normally holds about a week before his Dec. 23 birthday, but a palace official said Monday he will still appear before well-wishers for his annual birthday greetings to wave to them three times from a palace verandah.

He is also scheduled to wish visitors a happy new year on Jan. 1, but may appear at the window fewer times than usual, the palace official said, on condition of anonymity in keeping with imperial protocol.

Akihito's court physician issued an unusually candid statement earlier this month about the monarch's symptoms. Akihito underwent an operation for prostate cancer in 2003, but the palace statement suggested his latest ailments were brought about by worry.

"His majesty has felt mental stress over many matters," Shingo Haketa, the head of the Imperial Household Agency, said in explaining the statement.

He did not specify what matters were bothering Akihito, but according to several Japanese media reports, including one in The Asahi, a major newspaper, Haketa suggested that uncertainties over who his successor will be were among them.

Succession has for years been a major source of concern for Japan's royal family, which is the world's oldest hereditary monarchy.

Under a post-World War II law, only males are allowed to assume the throne. Akihito's eldest son, Crown Prince Naruhito, is next in line, but has no sons of his own. When he and Crown Princess Masako had a daughter, a movement heated up to change the law to allow the girl to succeed her father.

That died down when Naruhito's younger brother, Prince Akishino, had a baby boy two years ago.

But tabloids have speculated that a rift has developed between those who support the boy and those who believe the girl, now 7, should be allowed to reign.

Akihito has been careful in his public statements not to take sides.

High expectations and the rigors of life in Japan's tradition-bound palace have taken their toll on its members.

Though no longer revered as living gods, members of the imperial family live tightly regimented lives and are rarely allowed candid moments in public, making their appearances highly scripted and giving them little time to relax.

Masako, the emperor's 45-year-old daughter-in-law, withdrew from most of her official duties and appearances several years ago because of depression attributed to the difficulties she felt adjusting to palace life and the pressure of expectations that she would bear a son to continue the imperial line.

Empress Michiko, 73, has also often complained of stress and suffered a breakdown in the early 1990s that rendered her unable to speak for months. Last year, she suffered bleeding from the walls of her intestines, nose bleeds and mouth ulcers.

The palace said those illnesses were also related to stress.