Inside Expo '85, a 250-acre, $450 million mini-city north of Tokyo that opens today after more than seven years of preparation, there is a spirit similar to Disneyland's.

The Buck Rogers architecture, robots and giant-screened movies that dominate the fair are meant to be plain old fun. But organizers say they also will foster a passion for technology and other learning among visitors, especially younger ones.

"Our hope is that children who come here will think 'ah, science is a wonderful thing,' and devote their lives to it," said Yasuhiro Kada, assistant secretary general of the association organizing the fair.

Industrial spies can stay home. There is hardly any true front-line technology in the 56 pavilions; according to Kada, it was kept out on purpose. It takes an expert to get excited over a prototype large-scale integrated circuit (there is not a single representative of this key field of international competition on display). But most everyone feels something on seeing a robot wave a samurai sword or sketch a portrait.

In many ways, the choice of displays underscores a traditional Japanese strength in industry: the ability to find practical applications, whether for a child's room or a factory floor, for previously developed technology and take them to the world to sell.

Forty-seven countries, including the United States, will host pavilions at Expo '85, situated 35 miles northeast of Tokyo. Japan has worked to make it a fair for all nations, subsidizing the operation of the pavilions of some of the 37 non-industrial countries taking part.

Nonetheless, the Tsukuba expo is likely to be recalled as a mammoth display of hard sell and one-upsmanship by the private companies of Japan. More than 600 of them are here, running from giants such as Mitsubishi and Hitachi to ones little known abroad such as Suntory beverage group and Ueshima Coffee Co.

The idea for the fair originated in the mid-1970s, when government was concerned that industrial pollution and the oil shock were eroding people's faith in technology and the future. Kimiwo Fukushima, chief of the government pavilion and the man widely regarded as the fair's initiator, recently told an interviewer, "We needed a grand celebration. How about an expo?"

Today, it has become a national goal. Crown Prince Akihito is honorary president. It is hard to find a Tokyo subway car with no ad for it. Round-the-clock vigils for the first tickets began at the gates days ago, and thousands of schools are planning to send classes through for visits.

Organizers predict that when it closes in September, almost 20 million Japanese will have trouped through. Assuming there are no repeat visitors, that would amount to about one in every six people in the country. About one million foreigners are expected.

Visitors will be bombarded with Japanese concepts of service. Some 2,000 perpetually smiling women guides, dressed in sometimes silly futuristic fashions, sing out a chorus of "welcome" and "thank you" in pavilions across the fair.

In the "Robot Theater" of the Fuyo group, bevies of wheeled automotons done up as cartoon creatures move across a stage with choreographed precision. "From the point of view of entertainment, it can be called well done," said Kazuo Terakado, associate editor of a glossy Japanese science magazine called Newton. "But the technology is not particularly advanced."

The biotechnology exhibit drawing the most attention is a giant tomato plant, which its tenders promise will produce 10,000 tomatoes by the fair's end. The real focus of that field, chemical reactions in sealed containers, hardly would draw ooh's and ah's.

In the pavilion of electronics producer NEC, a "teleconferencing" table is in operation. Six people seated at it look at life-size TV images of six others at an identical table in Tokyo and gain the impression they are in the same room. It is riveting, but not a novel concept.

"If Expo had been geared toward display of Japan's most advanced technology, it would have been completely different," said Terakado. Still, here and there, there are technically significant exhibits to be ferreted out, he said. Among them:

* A robot that plays an organ, another that walks on two legs, and another that lifts barbells weighing 440 pounds, groaning in Japanese about the strain. All represent relatively advanced designs, being put to frivolous use.

* An experimental television producing a three-dimensional picture without the conventional colored glasses. It was made by the electronics giant Matsushita.

* The world's largest TV screen, the 80-by-130 foot "Jumbotron" of Sony Corp. Set up outside, it resembles an old drive-in theater and is visible across the expo.

* A 3-D film produced by the computer company Fujitsu that takes the viewer inside a star and inside a crystal of ice (almost every pavilion has a large screen show). The images are entirely computer-generated. "The theory behind it was already known, but still there was a lot of surprise when they were able to produce it," Terakado said.

The U.S. pavilion has taken artificial -- that is, computer -- intelligence as its theme. A succession of some of Expo's more intellectually challenging exhibits leads visitors through the history of computers and efforts to engineer in them processes of the human brain. One display computer composes jazz.

Use of the term "artificial intelligence," which by many accounts is still a long way off, raised a few eyebrows in the Japanese press. But according to pavilion deputy commissioner general Henry Gosho, one Japanese paper also noted that, whatever term is used, the U.S. pavilion gives the fair's best depiction of the concept. Technology's dazzle is grabbing the forefront at the expo. But the show's official theme is "Dwellings and Surroundings -- Science and Technology for Man at Home." In part, it was a gesture toward nonindustrialized countries, who would not be able to match Japan and the others in science.

So a Japanese publishing company's pavilion focuses on great archeological sites. France's has three rooms in a typical French home. Switzerland's looks at the role of alpine water in industry, Tonga's has a collection of native seacraft, and Suntory's is devoted to birds.

The Japanese government put almost $280 million into building the expo, most of which it will not get back. Much of that spending is aimed at promoting industrial development in the Tsukuba area after the the fair shuts down six months from now.

Every structure save the soaring white towers that carry high-tension wires will be razed and factories put up. Streets and sewers will remain, however, forming a basis for a new industrial park. According to Kada, more than 10 companies already have decided informally to set up there.

The government also hopes the fair will help put on the map the "science city" it inaugurated nearby in 1980. Currently, almost 50 laboratories and research institutes are located there, but it retains the sterility of many planned communities. Some employes see a tour there as an ordeal.

The government also used Expo as an excuse to pry loose about $1.8 billion in development funds for new highways from Tokyo into the area, sewers and new dock facilities at Ooarai port to facilitate shipments from Hokkaido Island.

Expo also has generated a boom in the local hotel industry, although many of the newly built rooms will be torn down with Expo. Among them is "Wagon City," 50 rail cargo containers modified into family-sized accommodations, complete with color TVs, bathrooms and tatami mats.