Now you see it: Toyo Ito's pavilion in Hyde Park

'I feel betrayed," Toyo Ito says, "as soon as I see my work completed." There is no satisfying some architects. Ito is the designer of some of the world's most magical and refined buildings, mysterious marriages between the solid and the transparent, the substantial and the ethereal, the sensual and the coolly technological. A master of lighting effects, he created a modern masterpiece with his Tower of Winds (1986), near the central railway station in Yokohama, Japan. By day, this tall, circular sheath of aluminium serves as a ventilation shaft for an underground shopping mall; by night, atmospheric changes and the sounds of the city transform it into a mesmerising light show, a kind of architectural firework. It is truly beautiful.

Ito has now teamed up with Cecil Balmond, the European chairman of engineers Ove Arup, to design this summer's garden pavilion for the Serpentine Gallery in London's Kensington Gardens. Expect to be delighted. Opening at the end of this week, the pavilion is the third in a sequence of experimental structures - following those by Zaha Hadid (2000) and Daniel Libeskind (2001) - commissioned by this spirited public gallery. It will host a summer-long series of events, including talks on architecture, a programme of films from the silver jubilee year, 1977, and the BBC Poetry Proms. It will also contain its own cafe, open until 10pm, when the structure will be lit up and, undoubtedly, at its best.

The 2002 Serpentine pavilion is a happy exercise in asymmetry and wily maths. A steel-framed structural enigma, it is nothing less than an attempt to blur the boundaries between walls, floor and ceiling, and between interior, exterior and sylvan setting. Given all this, why should Ito feel "betrayed" whenever he sees one of his subtle designs completed? Because this highly talented yet self-deprecating designer has been trying to achieve the impossible ever since he took up architecture instead of baseball in 1965, the year he graduated from Tokyo university.

Baseball is a reminder that Ito is a child of the second world war. Born in 1941, he witnessed what proved to be a paper dynasty vanish in firestorms and atomic clouds. All that was solid melted into air, to be replaced by a new, technologically driven society underpinned by Marshall Aid, Hershey bars and baseball. Civilisation, and architecture, no longer seemed substantial. And yet as Japan began to recover and modernise, its architects began to create some of the most overtly heavyweight buildings anywhere in the world - sumo architecture.

Ito's work is in part a meditation on and a reaction to this crushing tide of brutalist concrete design. His dream, since he set up his own practice, Urban Root, in 1971, has been to create an architecture, and an experience of the city, that is as much virtual as it is physical. He has long spoken of "concept buildings", of buildings that try their damnedest not to be enduring monuments. If you have had the good fortune to see his new Mediatheque at Sendai lit up at night, you will surely feel with me that he has come as close to realising his dream as any architect can working with current technology. This is a building of opportunities, a popular meeting ground of library, art gallery, film studio and cafe that is a model for what we could do in so many of our crowded city centres. Screw up your eyes and it almost vanishes.

In the Serpentine pavilion, Ito plays a game, with Balmond providing the mathematical and logical framework, in which solid and light elements are balanced on an architectural knife-edge. Balmond says that, notionally at any rate, the roof, walls, windows and doorways are a sequence ofgeometrically generated planes that "appear to be infinitely in motion". This is a non-linear architecture, "resulting from a design process where structure and surface amalgamate".

Given an unlimited budget, Ito and Balmond explain, the many triangles and trapezoids that make up the structure could be replaced by different types of glass, or grass, water or illuminated panels: in fact, any material safe to stand on, touch or look at. In its "virtual" state, the pavilion would be the kind of building that would find its true home in one of Italo Calvino's fictional Invisible Cities. In the real world, it does its best to suggest what a virtual building might be.

Architecture is undergoing the birth pangs of a new revolution, as Ito and his generation attempt to reach beyond the stars, while the tools to do so remain tantalising out of reach. (This is much as it was just after the first world war, when, for example, Mies van der Rohe displayed designs for crystalline, almost filigree skyscrapers, buildings impossible to realise at a time when most cars still resembled horseless wagons.) In Los Angeles, Frank Gehry's Walt Disney Concert Hall, a heavy-duty steel construction wrapped in a waltzing skin, is doing its unlevel best to deny its own solidity. This fascinating building marks a transition from 20th-century structural know-how to ideas that stretch, as yet awkwardly, into the virtual territory of the 21st century.

Balmond has worked with some of the great names of modern architecture, including James Stirling, Rem Koolhaas and Daniel Libeskind. Like Ito, Koolhaas and Libeskind are trying to liberate architecture from its solid base, to push a resistant spatial and material boundary. Balmond has been, in effect, the joint designer of the new port terminal at Yokahama with Foreign Office Architects, and, with Libeskind, of the Spiral gallery for the Victoria and Albert Museum, and he is an associate of Koolhaas's Office for Metropolitan Architecture.

Balmond agrees with Ito's belief that contemporary architecture can and should be "non-completing, non-central and synchronised with nature and urban spaces". By which, the Japanese architect means new forms of buildings might be open to change, free-spirited in structure and plan and less in-your-face than they have been. In one sense, Ito is looking back to the values of traditional Japanese architecture.

Ito's goal is perhaps an impossible one. Most clients and many architects want buildings that will stamp their name, their wealth and their vanity on to streets and cityscapes. But Ito's Serpentine pavilion is a clue to what buildings could be if only we let go a little and blurred the gap between architectural fiction and urban reality.

· The Serpentine Pavilion, London W2, opens on Friday. Details: 020-7402 6075 or www.serpentinegallery.org