In a traditional ceremony of the world's oldest monarchy, Prince Akishino, the newborn's father, wrote the name "Hisahito" on handmade paper at the hospital where the six-day-old is staying.

Princess Kiko, the boy's mother, then marked on another paper an umbrella pine tree, a unique royal symbol which Hisahito will carry for life and appear on everything from his official papers to tableware.

The pieces of paper were then put inside a box made of paulownia wood and placed next to the pillow of Hisahito -- the first Japanese royal to be born outside the palace in a private hospital.

 Hisa translates as serene in Japanese. Hito, which means virtuous, usually comes at the end of boys born to the imperial family.

"The name carried the wishes of the prince and princess that the new prince would have a long and serene life," an Imperial Household Agency official said.

The character for Hisa can also be read in combination with other characters as Yukyu, which means eternity, or Yuzen, which means serenity.

"As in Yukyu and Yuzen, the character has a meaning that one is 'perennial' and 'calm,'" the palace official said.

Akishino had considered possible names since July, consulting with scholars of the Japanese language and Chinese characters, which are used in Japanese. He finally picked the name Monday morning, according to Jiji Press.

The boy, born on September 6, is third in line to the Chrysanthemum Throne, after Crown Prince Naruhito and his father Akishino.

He is the first boy born to the royal family since his father -- who is also known as Fumihito -- in 1965.

Kiko's surprise pregnancy delighted conservatives as it came as the government looked set to introduce female succession to end the looming succession crisis.

 Kiko and the baby were both in good condition and expected to leave hospital in about a week.

Kiko also has two daughters, Mako and Kako, who were born in 1991 and 1994.

Naruhito and Crown Princess Masako, a former career woman who was under intense stress to produce male heir, have one child, four-year-old Princess Aiko.

Agence France Presse