Kenyan and Japanese scientists today announced discovery of teeth and jaw bone fragments that are 15 million years old and could be the remains of a very distant ancestor of man.

The announcement was made simultaneously here by Richard E. Leakey, director of the National Museum of Kenya, and in Japan by Hidemi Ishida, leader of the Japanese team and professor of primatology at Osaka University.

Leakey said the site in northern Kenya, which he described as unusually prolific, had yielded the bones of a hominoid species known as Kenyapithecus. He added:

"I can't tell you at the moment whether Kenyapithecus is going to turn out to be an incipient hominid, but I can tell you we have got a site where I think we are going to get the evidence to answer this question. It may be possible to make a definite statement of what was happening in Africa 14 or 15 million years ago in terms of this ape ancestor story.

"It's a tremendously important turning point in our science. There's a potential for getting the component part that will tell us what it is."

The initial discovery was made by Martin Pickford of the Kenyan team on Oct. 15, the final morning of the three-month field trip. This potential treasure trove of fossils was only about two miles from the expedition's camp. Expedition members had passed it daily on their way to another site a 2 1/2-hour walk away that has yielded a human-like upper jaw estimated to be 8 million years old.

The scientists discovered 22 teeth and jaw fragments scattered over two acres in one day. They also picked up about 100 fossilized bones of crocodiles, turtles, elephants, rhinoceroses and early relatives of today's antelopes. Existing sediments and the presence of water-related reptiles indicate that the site was once on the edge of a lake.

The area, which is about 300 miles north of Nairobi on the western edge of the Great Rift Valley, is now flat and treeless. Because it is virtually uninhabited, the scientists had no qualms about leaving further finds for another field season.

Leakey said that because of the high quantity of bones already found, further digging might yield some Kenyapithecus limb bones or even a skeleton. Until now, African hominoid fossil finds from this time period have been sketchy.

Until last year, Kenyapithecus was discredited as a species in its own right. The first example of one emerged when the late Louis Leakey, Richard's father, unearthed four teeth dated at 14 million years on the shores of Lake Victoria in 1961. He said that the teeth had characteristics that were more akin to humans' than to apes' ancestors.

In 1965, other scientists suggested that Kenyapithecus was merely an African example of the Asian Ramapithecus, which was at that time believed to be a possible ancestor of man. Then, last year, the remains of a Ramapithecus skull discovered in Pakistan showed that the species is an ancestor to the orangutan. Orangutans are not found in Africa. The new Kenyan site could determine the true place of Kenyapithecus in terms of human ancestry and the connection between early African and Asian apes.

When asked if the 15 million-year-old teeth and jaw fragments could be linked to his most famous find, a skull put at nearly 2 million years old, Richard Leakey replied that there was too big a time gap.

The skull, known by its catalog number of 1470, has a brain case large enough to demonstrate that a human-like creature with a relatively high level of intelligence lived on Earth nearly 2 million years ago. Leakey said additional funds are being sought to mount a major expedition to the new site in 1984. He said each field season would cost about $100,000.