Sandwiched between old residential apartments in the capital’s central Minato Ward is the Arimaston Building, an eccentric collage of individually patterned concrete slabs piled upon each other as if by happenstance.

It’s the work of Keisuke Oka, who has been building the unfinished structure mostly single-handedly, improvising as he goes. Some affectionately call him Mita’s Antoni Gaudi, in homage to the building’s address and the radically original Catalan architect known for his fantastical style.

It’s become something of an attraction in the bustling office district interspersed by shiny high-rises and restaurant chains. Curious onlookers stop to take photographs of the peculiar gray mansion that has been featured in art exhibitions and magazines for its distinctive design and story, one that reflects Oka’s own struggles as an architect searching for his brand of expression.

Perhaps it’s a sense of purpose the building evokes amidst the capital’s scrap-and-build culture that draws people to its entrance.

“I want to make something lively, not those lonesome buildings standing in big cities,” the 53-year-old self-builder says on a recent afternoon in his future home, which he has been putting together over the past 13 years.

He admiringly runs his hands over the concrete walls he made casting 70-cm blocks of concrete at a time — the most he can manage to haul — using domestically sourced cement, sand and gravel. Various objects and plants have been used to imprint the molds, giving each slab its own character.

Oka shows off a doughnut-shaped concrete object he made after being inspired by giant stone coins used on the Micronesian island of Yap. | YOSHIAKI MIURA

“Unlike the concrete used in most buildings in Japan, this is supposed to last 200 years, according to experts,” Oka says, explaining his method of mixing cement with less than 40 percent water compared with the nearly 60 percent commonly used in commercial buildings.

Higher water content enhances the concrete’s workability and translates to a smoother surface at the expense of reduced strength. Buildings in Japan are said to have a shelf-life of around 50 years. By lowering the water-to-cement ratio, Oka’s concrete is much stiffer and durable, giving it a substantially longer life span.

For Oka, Arimaston Building — the name being a playful concoction of the Japanese words for ant (ari), trout (masu) and kite (tonbi) — is the antithesis of the faceless buildings and homes being mass-produced by developers and house-makers.

He recalls a lecture by the late architectural historian Hiroyuki Suzuki that left an impression. Referring to the teachings by the Victorian art critic John Ruskin, Suzuki had said “things made halfheartedly are ugly, while those made with love are beautiful.”

“I thought that was it, that’s what I need to do,” Oka says.

Born and raised in Chikugo, a semiurban city in Fukuoka Prefecture, Oka spent his 20s traveling around Japan sketching the nation’s various architectural offerings while working at construction sites in Tokyo.

Over the span of a decade or so he took on various tasks, from moving earth and assembling scaffolding to setting reinforcement steel and molding and casting concrete.

While that gave him a broad understanding of how to build, he also experienced the reality facing construction workers, who seemed to be risking their health and well-being for the city’s massive infrastructure needs.

“I’ve worked at many building construction sites, but they really cut corners, almost to a criminal level,” he says. Safety and workmanship were typically sacrificed for speed and lower labor costs, he says, as rank-and-file workers dealt with hazardous substances.

The unfinished Arimaston Building in Tokyo’s Minato Ward is being assembled from 70-cm blocks of concrete made with domestically sourced cement, sand and gravel. Various objects have been used to imprint the molds, giving each slab its own character and texture. | YOSHIAKI MIURA

Oka was forced to stop working for around 18 months in his early 30s after his hands began to shake uncontrollably. A doctor said he suffered multiple chemical sensitivity.

“These arrogantly towering skyscrapers are erected at the cost of somebody’s health,” he says.

“There’s no love there. And that inspired me to build on my own.”

A first-class licensed architect, Oka hit a slump in his 30s. Temporarily unable to work due to health issues, the death of Yasuo Kurata, the founder of the Takayama Architecture School and Oka’s mentor, dealt another blow to his motivation. He considered abandoning architecture for good, he says.

This was, however, also around the time Oka got married. His wife, then a cabin attendant at Japan Airlines, convinced him to embark on what would become a life-changing mission.

“She said ‘you’re a first-class architect and have experience working at construction sites, so why not build our home?’ And without thinking too much, I agreed.”

They began looking for affordable property and eventually negotiated the purchase of a 40-square-meter plot of land in Mita for ¥15.5 million — a considerable discount from the original offering price of ¥65 million. For inspiration, Oka visited the self-made Sawada Mansion in Kochi Prefecture, often likened to Hong Kong’s now-demolished Kowloon Walled City for its maze-like, unpredictable design.

Construction commenced in late 2005. Oka was 40 by that time. He began digging a 3-meter-deep basement with the help of friends, shoveling out around 250 tons of dirt over the span of one year. He bought reinforcing steel and wooden boards for molds at home improvement centers and began casting cement blocks.

He kept the concept strictly improvisational, drawing from his days as a Butoh dancer studying under Yukio Waguri, a disciple of the legendary founder of the art form, Tatsumi Hijikata.

He had a general plan for the building: four stories high with a basement. But mostly, he decided on the go.

There is a pillar made from piles of thinly layered concrete bricks with varying textures. One wall is decorated by imprints of a branch he found lying near the Takayama Architecture School in the mountains of Hida, Gifu Prefecture, where he now teaches in the summer.

There are numerous crevices and holes along the walls, floors and ceilings, openings that may eventually become windows and skylights.

Apart from the odd jobs he takes on here and there, Oka is devoted full-time to his project. Besides a few T-shirts he purchased recently, he says he hasn’t bought clothes for himself in years. He remains in debt, and he and his wife split expenses so his financial condition doesn’t impact her lifestyle.

Oka, medium-built with cropped hair and an easy smile, doesn’t come across as especially media-savvy. But he says there is a reason he seeks publicity.

Keisuke Oka, a 53-year-old architect, has been working on the Arimaston Building in Tokyo’s Minato Ward for the past 13 years, mostly on his own. | YOSHIAKI MIURA

In 2009, he was notified that his building was in an area designated for large-scale redevelopment that will raze the neighborhood. Oka consulted a lawyer friend who warned him that, realistically, the only way to resist may be to launch a PR campaign to convince developers and relevant authorities of the building’s cultural significance. He took that advice to heart.

While he remains relatively unknown overseas, in Japan he is recognized among art and architecture circles as an edgy architect with a distinct philosophy. He recently received Takeshi Kitano, arguably Japan’s most famous comedian and filmmaker, into Arimaston Building for Kitano’s satellite-TV program. He was featured by cult manga artist Hideki Arai, and in April Oka published his first book chronicling his life, titled “Babel!” in reference to the biblical Tower of Babel. He was part of Mori Art Museum’s exhibition on Japanese architecture that wrapped up on Sept. 17.

His efforts may be bearing fruit. He says officials in charge of the redevelopment have suggested that his building may be spared from demolition. But nothing is set in stone and Oka isn’t convinced. He has seen how big money sucks the life out of design and those who painstakingly labor to realize it.

And he has good reason to be protective.

“If I didn’t decide to build this, I’m sure I wouldn’t have been able to make sense of my life and all my endeavors,” he says.

“Everything I’ve done would have scattered away.”

Like the cement locking together his brilliantly crazy invention, “this has been the adhesive,” he says.