The day-long ceremonies at the palace had been scheduled for April but were postponed because of the coronavirus pandemic and have been scaled back as infection keeps rising, although Japan has escaped the explosive outbreak seen in many other countries.
In front of attendees, most of whom were wearing masks, Akishino said he was pondering the responsibility of Crown Prince.
Akishino, 54 is one of just three heirs to the throne along with Hisahito, 14, and Prince Hitachi, 84, the younger brother of Emperor Emeritus Akihito, who stepped down last year in Japan's first abdication in two centuries.
Under Japanese law, only males can inherit the throne, so Naruhito's only child, 18-year-old Princess Aiko, is ineligible.
Moves to amend the law lost steam when Akishino's wife bore a son, Hisahito, in 2006.