TOKYO, SEPT. 4 -- Fujitsu Ltd., Japan's largest computer maker, beat International Business Machines Corp. to the punch today in announcing a powerful new mainframe computer that it says is the fastest in the world.

Rival IBM, the world's leading computer manufacturer, was expected to announce a similar machine on Wednesday.

Both companies are depending on their new generation of mainframes to maintain their technological and competitive leadership in the high-end computer market.

Fujitsu officials refused to announce the maximum calculating speed of the series, dubbed the M-1800, saying such figures don't reflect actual operating conditions. But the company said its top-of-the-line model delivers "the fastest processing speed in the world" for mainframes.

Unlike supercomputers, which are designed to make repetitive scientific calculations, mainframes must be able to handle a wide range of general business, database and communications information at the same time, and therefore are slower.

Toshio Hiraguri, general manager of Fujitsu's Computer Systems Group, said the M-1800 line outperforms mainframes recently announced by Hitachi Ltd. and NEC Corp. by at least 10 percent.

IBM officials decline to comment on Fujitsu's announcement.

Analysts say the new IBM top-of-the-line computer, code-named the Summit, will have six linked processors and run at a maximum speed of about 230 million instructions per second, or MIPS. IBM's current six-processor mainframe, its most powerful machine, runs at about 112 MIPS.

The fastest computer in the Fujitsu M-1800 series, the model 85, is the first in the world with eight processors and is about 28 percent faster than Fujitsu's six-processor model 65, Hiraguri said.

NEC in July announced a mainframe that it says is capable of 500 MIPS. But it uses a different operating system than IBM, unlike Fujitsu and Hitachi, making a direct comparison of processing speeds difficult.

The M-1800 series achieves its speed with special high-speed logic and memory chips, high-capacity fiber-optic connections and sophisticated batch processing of data.

The new IBM computers are expected to have similar features.

Fujitsu officials denied that they timed their announcement today to take the wind out of IBM's sails. "We've been planning to begin shipments next April, and our customers have been asking us to make the announcement as soon as possible," an official said. "So it's just a coincidence."

The computers also will be sold

overseas under the Siemens name, and their technology is likely to appear in mainframes produced by Amdahl Corp., a U.S. company partly owned by Fujitsu.

Amdahl and Hitachi generally have undercut IBM's mainframe prices and have eaten into its sales in recent years, although IBM still has about 87 percent of the U.S. high-end mainframe market, says Computer Intelligence, a California research firm.