Crown Prince Akishino, the younger brother of Emperor Naruhito, said ahead of his 58th birthday on Thursday that a review of official duties is needed as imperial family members age or leave the royal house.

"It would be difficult to pass on (official duties) if the number (of imperial family members) decreases," the crown prince told journalists in Tokyo earlier this week.

The Japanese imperial family has been shrinking as female members are required to give up their royal status upon marrying commoners under the 1947 Imperial House Law. The current total of family members stands at 17, with 12 being women.