In a strictly offical sense. Japan has maintained no embassy in Taiwan since it recognized mainland China six years ago and moved its diplomatic mission to Peking.

So what happens to the Japanese businessman seeking trade contacts here? Or to one Japanese tourist who loses his passport? or to the Taiwanese who wants a visa to vist Japan?

The answer is that all would go to an office building at 43 Chi Nan Road and discuss their problems with the people at something called the "Interchange Association." It's no big secret that this is the Japanese Embassy in disguise. It is strippeed of official symbols and there isn't a government seal in sight but the Interchange Association does just about everything an embassy does.

Other countries deal with Taiwan through organizations with similarly nondescript names. For the British there's the Anglo-Taiwan Trade Committee. The Germans have their Deutsches Kulturzentrum and the Spaniards their Cervantes Center. The people from down under operate through the Australia Free China Society.

All were established when those countries recognized Peking as the real capital of China and were required by the Communist government to abandon official contacts with Taipei - or at least to act as if they had abandoned them. They set up unofficial ostensibly private associations with names that John Le Carre might have invented to mask a British inteligience network. Some of the disguises are elaborate but it is doubtful that many people are misled.

On Jan. 1. the United States will follow suit and establish its own diplomatic fiction here. No one is saying exactly what it will look like but the changeover may be no more complicated that the removal of plaques that say "Embassy of the United States." It is even possible that the staff will remain intact with foreign service officers becoming private citizens at least in name.

At one time more than 100 countries recognized Taiwan as an independent government and maintained embassies or consular officers here. By Jan. 1 the number will be down to 21. Most are small nations from Central and South American or Africa. The three important holdouts here are South Korea, Saudi Arabia and South Africa.

Although the departures hurt Taiwan's pride it can be argued that the loss of recognition doesn't make a great difference in practical terms. Since Japan pulled out trade between the two countries has continued to boom. Taiwan exported more than $1 billion worth of goods to Japan last year four times the amount in 1972 when diplomatic relations were severed.

Japan's emissaries in the Interchange Association are reluctant to talk with reporters about what they do. But one aide agreed to an interview on the condition that his name not be published and that certain questions dealing with political issues remain unanswered.

"We do just about everything that an embassy does," he said. The association arranges trade contacts helps with visas and puts on cultural events. Does it report to Tokyo on political developments in Taiwan? The unofficial official said he would prefer not to comment.

The Interchange Association staff resembles a minigovernment-in-exile. The director is Akira Nishiyama who used to be ambassador to South Korea. The 12 staff members all are on leave from the Japanese foreign ministry and five other government ministries. They will return to official careers in Tokyo when their terms are up here.

The official issuance of visas is supposed to be forbidden so another polite fiction has been constructed. The Interchange Association receives applications for visas and according to the aide mails them to Hong Kong where they are stamped and mailed back to Taipei. Some independent observers suspect the visas actually are stamped here and held up for a week for the sake of appearances.

Official contacts between the Interchange Association and the Taiwan government also are prohibited for fear of arousing Peking's wrath. And so another arrangement has been perfected. When the association has a problem requiring official attention it contacts a Taiwanese organization with the marvelously vague title of the "Association of East Asian Relations." It is ostensibly a private unofficial establishment but its staff is said to be drawn from the Nationalist government's Ministry of Foreign Affairs.

That Association says the Japanese emissary "transmits our letters to any relevant agency of the government. We are not allowed to contact the Foreign Office directly." Usually, he said, the matters involve trade investments and technical assistance.

Many of the quasi-embassies here are far less formal than Japan's Interchange Association. Somes merely maintain informal trade contacts and help out tourists leaving visa business to airline offices.

The Australia Free China Society is a good example. It was set up five years ago when Australia severed relations and is primarily the creation of a New South Wales politican. Douglas Darby whose personal affections and political sympathies lie with Taiwan.

The organization avoids the visa problem completely. If a Taiwanese wants to visit Australia he applies through Qantas or Cathy Pacific Airlines and the visa is stamped at the Australian Commission in Hong Kong. An Australian wanting to visit Taiwan applies through the Taiwan Travel Service in Sidney and receives a certificate which is the equivalent of a visa.

"Mainly we just help out Australians who come here and get in trouble," says James Cheng, a Taiwanese who directs the Society's staff of five here.

A young Australian recently got himself jailed for carrying marijuana in Taipei and Cheng sees that he gets a supply of vitamins and books in his prison cell. Another Australian tourist was killed in a traffic accident and Cheng arranged for the coffin to be flown back home.

"If an Australian loses his passport here where would he turn if there were no organization like ours?" asks Cheng.