Hidden Wonders | Tokyo's Twisting Concrete Masterpiece Complete After 20 Years

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The building on Hijirizaka slope in Tokyo's upscale residential area of Minato Ward is hard to miss. Made of reinforced concrete, it twists and turns, with walls and ceilings covered with enigmatic patterns.

Named Arimaston Building (蟻鱒鳶ル "ant trout black kite building"), this striking structure feels more like a sculpture than a conventional building. Architect Keisuke Oka spent 20 years creating this self-built masterpiece.

"I can't believe I created something like this," Oka said, gazing at his work. The scaffolding and coverings were removed in late October. With a broad smile, he continued, "I'm amazed that something beyond my expectations has come to life."

*Architect Keisuke Oka and his team look up at the building on October 24 after the scaffolding was removed. (©Sankei by Yuta Yasumoto)*

Innovative Design and Durability

Construction began in 2005. With four stories above ground and one below, the building occupies just 40 square meters — a small residential space. Oka, who is also a dancer, infused the project with a sense of improvisation.

The initial concept was vague, just a general vision. Oka worked ad-lib, incorporating various materials such as food trays and plants into the formwork, and designing windows with intricate polygonal shapes. Friends who helped also contributed their creativity, further fueling the project's improvisational nature.

*The rain gutter installed on the outer wall is transparent and forms a spiral. October 24, Minato Ward, Tokyo. (©Sankei by Yuta Yasumoto)*

The architect used a concrete mix with reduced water content, making it so durable that an expert claimed it would last for 200 years. Thanks to Oka's careful craftsmanship, the surface gleams with a strange luster, and the design exudes a mystical aura.

Future Ambitions

However, construction is not quite over. Due to a redevelopment project in the area, the building will be moved 10 meters (about 33 feet) southeast in the summer of 2025. This will be done using the traditional Japanese method of "hikiya," where a building is lifted and slid onto rollers to be relocated.

*Polygonal windows punctuate the concrete walls. (©Sankei by Yuta Yasumoto)*

Nevertheless, the building itself is complete. Although it seemed premature, I asked Oka about his next goal.

"I want to collaborate more seriously with others to build architecture in a fun way," he said. "In the past few years, I've met some fascinating people, and I want to start new projects that will bring even more excitement."

It may not be long before we see more Arimaston Buildings, each bringing a unique character to the streets of Japan.