When Satomi Nakamura uses her cellphone, she has to be extra careful to take frequent breaks. That's because she isn't just chatting. The 22-year-old homemaker has recently finished writing a 200-page novel titled "To Love You Again" entirely on her tiny cellphone screen, using her right thumb to tap the keys and her pinkie to hold the phone steady. She got so carried away last month that she broke a blood vessel on her right little finger.

"PCs might be easier to type on, but I've had a cellphone since I was in sixth grade, so it's easier for me to use," says Ms. Nakamura, who has written eight novels on her little phone. More than 2,000 readers followed her latest story, about childhood sweethearts who reunite in high school, as she updated it every day on an Internet site.

In Japan, the cellphone is stirring the nation's staid fiction market. Young amateur writers in their teens and 20s who long ago mastered the art of zapping off emails and blogs on their cellphones, find it a convenient medium in which to loose their creative energies and get their stuff onto the Internet. For readers, mostly teenage girls who use their phones for an increasingly wide range of activities, from writing group diaries to listening to music, the mobile novel, as the genre is called, is the latest form of entertainment on the go.

Most of these novels, with their simple language and skimpy scene-setting, are rather unpolished. They are almost always on familiar themes about love and friendship. But they are hugely popular, and publishers are delighted with them. Book sales in Japan fell 15% between 1996 and 2006, according to the Research Institute for Publications. Several cellphone novels have been turned into real books, selling millions of copies and topping the best-seller lists. "Love Sky," one of the biggest successes so far, is about a boy with cancer who breaks up with his girlfriend to spare her the pain of his death. It has sold more than 1.3 million copies and is being made into a movie due out in November.