Crown Prince Naruhito and Crown Princess Masako attended the coronation Tuesday of the new Dutch king at a historic church with members of other royal families.
Prince Willem-Alexander became the Netherland's first king in more than a century after his mother, Queen Beatrix, abdicated following 33 years on the throne.
The 49-year-old Crown Princess, who has long struggled with a stress-induced illness, was making her first official overseas trip in 11 years.
In addition to government officials and key political figures, members of royal families from about 20 countries around the world were among those attending the ceremony at the Nieuwe Kerk church, the site for Dutch royal inaugurations.
After the new Dutch king takes his vow, the royal families were scheduled to pose for photos and then move to the Royal Palace nearby for a reception.
In Tokyo, the Imperial Household Agency said Emperor Akihito sent a congratulatory telegram to the new king.
Ahead of the coronation, the Crown Prince attended a banquet on Monday organized by the Dutch queen, but the Crown Princess skipped the banquet to reduce her workload.
The Crown Prince went to the banquet at a national museum with Swedish Crown Princess Victoria.
The Crown Prince and Crown Princess are expected to leave the Netherlands on Thursday and return home the following day.