TOKYO (AP) _ A Japanese scientist believes moon rocks hold all the energy the world will need for centuries.

Within decades, fusion reactors using helium-3 extracted from the rocks and hydrogen from sea water could provide energy that is cheap, efficient and not radioactive, says Hiroshi Momota of the National Institute for Fusion Science.

Possible use of helium-3 as a power source in fusion reactors has intrigued scientists for years, but has generally been considered a challenge for the future. After several years of studying the idea for the Japanese government, Momota believes the future is now.

Moon rocks ferried to Earth could provide enough helium-3 to last 400 years, he estimates, and there is more of the resource on Mars and Jupiter.

''In the coming century, we will be in great need of energy, and moon rocks and solar power could be the only answers,'' Momota said.

Scientists have tried for years to re-create in a controlled manner the process of nuclear fusion - the melding of atoms under intense heat - that powers the sun and stars.

Late in 1991, a European team using two kinds of hydrogen, deuterium and radioactive tritium, managed for the first time to produce a significant amount of electrical power.

Tritium has serious drawbacks, however. It generates radioactive waste and requires massive and extremely expensive reactors that also could produce plutonium, used in nuclear weapons.

Helium-3 fusion would require much higher temperatures, but would not involve radioactivity. It could generate energy more efficiently and, once moon bases are established, at half the cost of nuclear power plants, Momota said.

''It's clean and it's cheap,'' he declared. ''It really is like a dream.''

With money from the Education Ministry, Momota worked out a conceptual design for a helium-3 fusion reactor he says could be ready by 2011. He estimates the cost of a prototype reactor at $350 million.

''It's a very interesting concept and we are looking into it,'' said Tsutomu Iwata, head of systems development for the National Space Development Agency, Japan's version of NASA. ''I think a moon base and commercial fusion could be realistic in another 30 years or so.''

At a meeting in Moscow in October, Momota and several American and Soviet fusion specialists issued a statement proposing a permanent manned lunar station to mine rocks for helium-3. They said it ''offers the first chance to gain significant economic returns from the exploration of space.''

Glenn T. Seaborg, a Nobel laureate in chemistry, and Paul H. Nitze, former presidential adviser on arms control, have appealed for U.S. government support of helium-3 research.

Writing in The New York Times, they said: ''We may be at the brink of a new era of energy technology - compact, safe, economical and producing virtually nothing but the energy we need. We should test it now and find out.''

Critics say establishing moon bases might be a waste of money.

''I know that there are several public and privately held companies in the United States that are actively pursuing this, but it is just one of many technologies that may or may not prove very promising,'' said Richard P. Macleod, president of the U.S. Space Foundation.

Momota and Iwata said Japan leads in helium-3 fusion research, but would be at a disadvantage trying to mine moon rocks.

Japan has only a fledgling space program, so cooperation with Washington or the former Soviet Union would seem logical.

But the Japanese fear instability has grounded joint projects with Moscow and are losing faith in Washington because some congressmen want to terminate Freedom, an international space station project Japan has joined.