Reiko Yuyama believes that adventures are there to be had in daily life without having to go out into the wilderness. In that sense, she says she might be "more of an adventurer than Christopher Columbus or Naomi Uemura," the late, great Japanese explorer and climber who disappeared on Mount McKinley in Alaska in 1984.

No, Yuyama is not a bungee jumper or a rebel graffiti artist -- her great urban adventure is "to go to expensive sushi restaurants by myself."

Writer and creative director Yuyama is neither a loner nor short of friends or family to dine out with. It is just that solo sushi sorties are her hobby -- and, like those who sail oceans or climb mountains, there's something very powerful driving her, too.

"Nowadays, women go by themselves to anywhere on earth," she says. "It is not that surprising if a female worker at a local post office takes a one-week vacation to go to, say, a rave party in Zambia or on a trip to swim with dolphins in the Bahamas.

"In town, women might stop by alone, for example, at their favorite sherry bar on their way home to relax for a while after an unexciting match-making party.

"But despite that, there remain some places in particular that are difficult for them to go to -- namely expensive sushi restaurants."

Speaking in an upbeat tone full of humor, 44-year-old Yuyama explains that she calls herself an "adventurer" because she dares to go to such places by herself. In fact, she reckons that she has visited a total of about 50 top-end sushi restaurants alone, mainly in Japan. Based on her experiences, she published a book in 2004, titled "Onna Hitori-zushi (Solo Woman Sushi)."

The act of onna hitori-zushi, Yuyama says, underlines the sushi restaurants' unique features. "Sushi restaurant culture has developed along with the corporate culture in Japan, which considered expensive sushi places as convenient venues for high-powered corporate entertaining. Naturally, those places have come to serve mainly male customers, especially men with power."

As a result, many of those establishments have come to play the role of exclusive salons, where regular customers and the chef develop a special bond. Those customers generally know a lot about sushi, and also know the prices, even though their host does not disclose them directly. "Women also go to such restaurants -- but generally they are accompanying those 'knowledgeable' men," Yuyama says.

As a result, in that environment, a lone woman diner would usually feel as out of place as a plate of fish and chips.

But Yuyama often goes by herself -- and only rarely with her husband -- and while there she enjoys her own form of "cultural research" as she eats. When asked her occupation by tipsy customers or an intrigued chef, she often replies that she is a Self-Defense Forces officer or a housewife, which makes those around even more confused.

What started Yuyama out on this voyage of hitori-zushi discovery was, she says, a bad experience she had in the late 1990s. That day she was exhausted after doing a long series of interviews, and was really looking forward to relaxing over a meal with friends. But unfortunately, none of her friends were free, so she decided to treat herself and go alone to an established sushi restaurant in Tokyo's smart Meguro district.

Though she was a newcomer there, Yuyama was hoping to have fun talking with the chef. But that night he was preoccupied with a group of two foreigners and one Japanese having business talks and she was almost completely ignored.

"With the foreign guests in the restaurant, the chef didn't seem to be interested in looking after a strange customer like me," she says. "I drank beer and sake and ordered several dishes of sushi, but then, when I was thinking about what to have next, a cup of tea was put in front of me. It was a sign that my dinner was over."

Unbowed, however, Yuyama stayed there another hour and ordered another sake -- but neither the chef nor any other staff showed any interest in her at all. It all left an unpleasant taste in her mouth, she recalls -- especially as her bill came to 20,000 yen.

Rather than being put off by that experience, though, Yuyama became intrigued by that world of expensive sushi restaurants -- "probably because I am a type of person who likes to rise to difficult challenges," she said with a hearty laugh.

Judging by her book, however, that bad experience in Meguro has not proved typical. In fact, many sushi chefs and serving staff she describes come across as being quite friendly -- if often surprised -- to be catering to her alone, and so what she writes is usually more about her taste experiences and the character of the places she has visited.

"Yes, that first experience was awful, but the service at other restaurants has generally far exceeded expectations," she says. "Also, I have noticed that the type of customers in these sushi restaurants has been gradually changing, and that is affecting their atmosphere."

Now, Yuyama observes, the number of business customers is fewer than before -- presumably as hard times bite into company expense accounts. Instead, she has detected a new species of customer emerging, namely "gourmet otaku" -- who are often pairs of men. "They are kind of amateur restaurant critics, who write about the places on their Web sites."

Along with that trend in changing clientele, and the growing numbers of women in groups she says she now notices on her sushi adventures, Yuyama feels that even solo woman are now not so often looked on as strange even at top-end eateries.

"But still, I believe, expensive sushi restaurants retain something of the atmosphere of a men's salon," she says, "and that may have something to do with the long tradition that sushi chefs are almost always men."

But not one to shrink from that challenge either, Yuyama has been busy enjoying herself as she has learned how to prepare sushi. Now, indeed, she even occasionally fills the role of a project called bijin sushi (good-looking woman sushi chef), which offers a sushi-cooking service for events and parties.

"I started that because I wanted to know how you feel if you work as a sushi chef behind the counter. I have learned what hard work it is to prepare each fish.

"Also, now I know that being able to make sushi beautifully in front of people's eyes requires a lot of experience and such a high level of skill."

Who knows, but one day Yuyama the adventurer may yet treat a solo male diner to an evening of sushi, sake and smalltalk -- without giving him strange looks or the cold shoulder, of course.