Downtown, in this West German city of executive suites, the Japanese have built an office center that rises, like an architectural jigsaw in angles of glass, slices of granite and layers of aluminum.

The structure was designed to get the Japanese and West Germans under one roof. They have not, and their failure to do so has become symptomatic of other, larger strains developing between these two great postwar trading powers.

Put up two years ago to house both Japanese and West German firms in equal proportion, the center was supposed to be a sort of monument to the friendship between the world's new economic miracle cultures. It was named the German-Japanese Center.

But few Germans ever took up the offer of residency. Today, only 5 percent of the building is occupied by West German tenants.

"This is actually a Japanese center," said Hideo Mizumoto, president of Marubeni's West German subsidiary, the trading company that paid $120 million for the eight-story building.

The rent, about two to three times the cost of neighboring office space, turned out to be too expensive for West German company budgets.Besides, the center's frills -- it is air-conditioned -- and its expansive design simply didn't appeal to conservative German corporate tastes.

West German corporations find that they differ increasingly with the Japanese on economic matters, too.

Although politically they are still on very friendly terms -- both nations share an avowed belief in laying claim to everyone else's markets -- West Germany and Japan are coming closer to serious economic blows, as competition increases with one another on their own home turfs.

The Japanese especially have begun to target West Germany, and Europe generally, penetrating certain markets with a ferocity that matches their assault on the American economy in the 1970s.

Japanese products accounted for 2.7 percent -- roughly $4.2 billion -- of West Germany's total imports last year. But this relatively small number masks the fact that Japan's foot-holds in West Germany rest in some of the fastest growing markets.

About two-thirds of all tape recorders and dictating machines in West Germany, for example, are made in Japan. The same is true of 3 out of 5 watches, 4 out of 5 pocket calculators, half of the portable television sets, most cameras and nearly all transistor radios.

West Germany automakers, as well, have looked on with deepened angst as Japanese models for the first time took more than 5 percent of the market last year, then accelerated past the 8 percent mark early this year.

In an unusual appeal in March, West German electronics manufacturers asked the Bonn government to impose restrictions against Japanese imports.

The government rejected the request. Among the major European trading nations, West Germany traditionally has been most resistant to calls for trade barriers against the Japanese. As one Bonn economics official said, "We have historically taken a liberal trade attitude because we are ourselves so dependent on trade."

Nevertheless, the pressure on Bonn is growing.

This quickening battle between America's two chief economic rivals might give U.S. observers certain pleasure. Both nations were, after all, rebuilt with U.S. assistance after World War II and since have come back to challenge American firms in world markets.

Their economies, distingushed by relatively low inflation and unemployment, are often singled out as examples of what to do right -- keep money tight, encourage investment, discourage credit, promote friendly labor-management relations -- against what Americans have been doing wrong.

While the economic policies of West Germany and Japan are similar, however, the way they do business is often very different. What they have most in common are success and an overriding confidence in their own abilities. m

The largest concentration of Japanese businessmen and their families in Europe -- about 4,500 representing about 240 firms -- is headquartered here in Duesseldorf. That is not much in a city of 1.2 million people, but the Japanese have made themselves noticed, giving rise to such nicknames for Duesseldorf as "Little Tokyo on the Rhine."

The German-Japanese friendship dates back to the last century when Japan, opening doos to the West, took interest in German philosophy, law, machine and art. Their World War II alliance reinforced the old ties and this has carried over to today.

"The Japanese prefer to work with Germans over Americans. The traditional friendship is still there," observed Herbert te Kloot, Duesseldorf's liaison to the Japanese community.

There are exasperations on both sides when doing business, however. "In Germany we are clear and open when we deal; the Japanese are not very clear," said Wolfgang Miethke, manager of Duesseldorf's economic development office.

Delays also come about as a result ofthe Japanese approach to management. The Western approach emphasizes individual input and initiative. In contrast, the Japanese stress consensus-building and a sense of participation by all members of the organization.

This means more Japanese are likely to get involved in making a decision which, in a West German or American firm, would be handled by just one or two people.

"We always have to talk to five or 10 Japanese who ask the same question," Miethke explained. "It is very annoying sometimes because you don't always see progress."

The Japanese prize their method because, they say, what is lost in administrative efficiency is made up in execution by the advantage of having everyone involved.