With a concrete Sears Roebuck facade and windows filled by trendy Gap jeans, Cambridge's Porter Exchange building seems to have all the elements of mainstream America.

But at this Mass. Ave. mall, American culture hardly penetrates the skin. Sears and The Gap may catch the eye of passing drivers, but inside, names like Kotobukiya and Sapporo draw the crowds.

In the eight years since it opened, the Porter Exchange has become New England's mecca of Japanese culture, drawing customers from New Hampshire, Vermont and Connecticut.

But despite the shopping center's success in attracting Japanese enterprises, the enclave developed mostly by chance.

In 1989 Masa Kuizumi left his job at Boston's up-scale Kyoto Japanese Steakhouse and decided to join several friends who were starting businesses at the Porter Square site.

Although the Asian entrepreneurs didn't start with great expectations, many say they have been enjoying tremendous success in recent years.

"We didn't expect this to happen," said Taisi, a chef who came to Porter Square with Kuizumi.

Taisi explained that his particular store, like many of its neighbors, is beginning to have problems serving all their customers.

"Now it's kind of reaching its maximum. There are only 15 seats here, we can't serve everybody."

Such popularity is due largely to Japanese and Japanese-American customers who find that the Porter Square mall offers the a fleeting taste of home.

Osamu Kaminuma moved from Tokyo to Lowell, Mass., two months ago.

As he was relaxing with his son outside the Kotobukiya grocery store last weekend, Kaminuma explained that the Porter Exchange shops are expensive but remain critical to his family.

"For me it's like I have to come just because it's Japanese," he said.

Kaminuma explained that he usually shops at generic American grocery stores, but makes trips into Cambridge every two weeks for specialty items like spices, sauces and rice.

Kaminuma admitted that another important reason for the trips into Cambridge was to please his young son, who was immersed in a newly purchased Japanese comic book as his father spoke.

But Japanese-Americans aren't the Exchange's only clients.

As young Japanese and Japanese-American customers sat gobbling up raw fish at one of the mall's two sushi bars Saturday, the atmosphere was riddled with subtle hints of American culture.

On the television screen hanging above cold salmon, squid and octopus, the North Carolina Tar-Heels were driving on the Virginia Cavaliers in one of the day's college football matchups.

"They almost always have a football or baseball game playing," said Mark W. Gailus '74, an Exchange regular and Somerville resident who was having lunch at the bar with his wife and son. "We usually come for the food-it's a nice change of pace," he said.

Gailus said that he never would have known about the Japanese enclave if he hadn't stumbled on it one day while doing other errands.

But while the Asian arcade may draw a relatively diverse group of clients, managers admit that is also a small group.

"We don't consider ourselves a destination," said Joe Roux, the Exchange's facility manager. "You either already know of us or you're from the neighborhood."

But Roux is quick to point out that the cluster of Japanese businesses is utterly unique.

"The only place that I could consider close to this is Chinatown," Roux said.

Indeed, while Japanese supermarkets may exist in Central and Union squares, nothing in New England parallels the mall's cultural concentration.

Roux also pointed out that the food at the Exchange is of a different quality than its competition.

"The food here is more traditional-it's all prepared in the building by hand," he said.

And according to Roux, such quality attracts a certain group of customers.

"I would say that our clientele is generally on the higher end of the income spectrum," he said.

Other described Exchange's clients in more practical terms.

"If you want to see the latest hot cars, just sit around this lot for a few hours," said a security guard who refused to give her name.