Toyota Motor Corp. has finally accepted an axiom of the global auto business: You can't become a world automotive power unless you build cars in the United States.

That is the significance of Toyota's announcement yesterday that it will start building mid-sized cars in this country by 1988, according to auto industry analysts and officials.

Toyota long has been Japan's largest auto producer, but among the three biggest Japanese car companies, Toyota has been the most timid in establishing a manufacturing presence in this country.

Toyota's past reluctance to enter the United States on its own as a manufacturer is understandable, analysts said. Even with "voluntary quotas" limiting the shipment of Japanese cars to this country, Toyota sold 557,979 cars and trucks in the United States last year -- more than any Japanese or other foreign auto maker doing business in the U.S. market.

With Japan's per-car production costs being $2,000 less than those of American rivals, Toyota's sales in the United States provided the single largest source of income for Toyota, analysts said.

But Toyota's joy ride began running into bumps in 1982, the year when Honda Motor Co. Ltd., Japan's third-largest car company in terms of sales, began assembling Honda Accord sedans in Marysville, Ohio.

Toyota at first seemed to ignore Honda's U.S. manufacturing efforts. But Honda last year combined imports with its American-made cars to sell 508,240 units in this country -- only 49,559 units less than Toyota's U.S. sales in 1984.

Toyota last year controlled 46 percent of its home market, versus Honda's relatively skimpy 9 percent market share in Japan. Toyota's worldwide sales last year totaled $23 billion in U.S. dollars, versus Honda's $10.2 billion.

Honda's 1984 U.S. sales performance surpassed Japan's No. 2-ranked Nissan Motor Co. Ltd., which sold 485,298 passenger cars in this country last year. Nissan opened a pickup truck plant in Smyrna, Tenn., in 1983 and built 100,500 trucks here last year.

Toyota executives were forced to take note. There was even speculation in the U.S. automotive trade press that Honda might some say overtake Toyota in the American market.

But few auto industry analysts took that talk seriously. They said that Toyota simply would follow Honda's example and use U.S.-built cars to get around import quotas and to maintain its position in the world's largest, most lucrative and most open automotive bazaar -- the United States.

Toyota took a tentative step in that direction late last year when it began building Nova passenger cars with General Motors Corp. in Fremont, Calif. Initial production at the joint-venture company, New United Motor Manufacturing Inc. (NUMMI), was insignificant in terms of numbers. But analysts expect NUMMI to turn out 30,000 passenger cars in 1985 and to increase that number to 250,000 annually by 1988.

Most of the NUMMI cars will be assembled for GM's Chevrolet Division. The remainder, about 50,000, will be built specifically for Toyota.

Nissan, Mazda Motor Corp. and Mitsubishi Motors Corp. also are following Honda's example. Japanese auto makers as a group could wind up assembling up to 900,000 cars a year in the United States by 1990, according to analysts.

Toyota's decision to start its own assembly operation "recognizes the fact that it can't afford to lose its grip on this auto market and says strongly that it intends to operate as a world auto power," said Arvid Jouppi of Arvid Jouppi Associates Inc., a Detroit-based marketing analysis firm.

Japanese auto makers "sell three out of four of their cars outside of the United States, but most of those sales are 'loss leaders,' " Jouppi said. "Japan's profit center [in auto sales] is the United States. Toyota realizes that."

Toyota and other Japanese auto makers also believe that they "will always face some kind of quotas" exporting cars to the U.S. market, said David Healy, an analyst with Drexel Burnham Lambert Inc. Quotas limited Japanese auto shipments to the United States last year to nearly 1.9 million and limits shipments for this year to 2.3 million.

"They are choosing to build here, even though it's more expensive for them to do so. But ironically, they are building cars in the United States cheaper than their competitors in Detroit," Healy said.

Other analysts pointed out another irony: GM ostensibly teamed up with Toyota in the NUMMI project to "learn" how to build small cars in this country at a reasonable production cost. But Toyota plans to build mid-sized cars -- one of GM's specialties -- without GM's help.

Toyota's U.S.-made line -- about 200,000 cars annually -- is expected to be a remake of its now popular 2-liter, 4-cylinder Camry family sedan.