YOKOHAMA, Japan -- Japan's Naoya Inoue announced his move up to boxing's super-bantamweight division Friday and set his sights on becoming undisputed world champion again, just one month after claiming all the bantamweight belts.

The unbeaten Inoue, nicknamed "Monster", became the first undisputed world bantamweight champion in half a century when he beat England's Paul Butler in December to add the WBO title to his WBC, WBA and IBF belts.

He said he had already decided to vacate his titles and take on a new challenge in the super-bantamweight division, with negotiations under way for his next fight.

The 29-year-old Inoue, who has a 24-0 record with 21 knock-outs, said he was moving up a weight class because there was "nothing left to do and no one I want to fight" in the bantamweight division.

"It will be a challenge against opponents who are bigger than me but this is the real fight coming up," said Inoue, who previously held world titles at light-flyweight and super-flyweight.

"I'm really excited about it."

Inoue knocked out Butler in the 11th round in Tokyo to become the first undisputed bantamweight world champion since Panama's Enrique Pinder in 1972.

He also became only the ninth undisputed world champion since the four-belt era began in 2004, and the first in the bantamweight division.

No man has ever achieved the feat in more than one weight class, but Inoue believes he can write a new chapter in boxing history.

"Even moving up to super-bantamweight, I still want to aim to become the undisputed champion," he said.

"To be undisputed champion in two weight classes would be a world's first and a fantastic achievement, and that's something I want to aim for."

- 'Final chapter' -

Inoue and his team would not be drawn on the identity of his next potential opponent, but he said he was willing to take on a top fighter straight away.

Uzbekistan's Murodjon Akhmadaliev is the current WBA and IBF world super-bantamweight champion, while American Stephen Fulton holds the WBC and WBO belts.

Inoue said there was "a lot of talent in the super-bantamweight division" and he is willing to bide his time to reach the top.

"It took me four years and eight months to go through the bantamweight division and I think it will probably take the same amount of time in super-bantamweight," said Inoue, who has said he intends to retire at 35.

"I think it's fair to say that super-bantamweight will be the final chapter for me."

Inoue's younger brother Takuma is also a professional boxer who formerly held the WBC interim bantamweight world title.

The older Inoue said he would like to see his brother challenge for one of his former belts once he has vacated them.

"It has always been my dream for us to both be world champions," he said.

"I'm vacating my titles and moving up to super-bantamweight, and I think that goal can become a reality in the near future."

© Agence France-Presse