Fumihito, in a news conference given before his birthday, said that members of the imperial court should always be thinking how they can "alleviate the physical burden" on the emperor, who turns 79 in December.
"It's not so easy to alleviate the burden on his majesty while respecting his own desire," Fumihito said.
Fumihito, the second son of Akihito and Empress Michiko, was joined at the news conference by his wife, 46-year-old Princess Kiko.
Fumihito said he thought his father decided to undergo heart bypass surgery in February "because he wanted to be able to perform his duties in as good shape as possible as Japan faces great difficulties following last year's Great East Japan Earthquake."
Asked when he realized his father was recuperating from the operation, Fumihito said it was around spring. The prince recalled it was then that he heard the emperor's appetite was recovering.