When Naoko Ito uses a public bathroom, she cringes in embarrassment at the thought other patrons can hear the sounds coming from her stall.

That’s when she turns to the Sound Princess.

Ito, like a rapidly growing number of women here, presses a device installed in public toilets to simulate the sound of water flushing — and mask the cruder noises of nature.

“I usually use the flushing sound when I go to a public bathroom, such as at a department store, because I get a bit self-conscious,” said Ito, a 60-year-old waitress.

The device — a curious mix of Japanese bashfulness and modern technology — is spreading rapidly through public buildings and has now become standard equipment for new construction.

Leading toilet producer Toto Ltd. has sold 500,000 Sound Princess units — Oto-Hime in Japanese — since 1988, and the company says orders surged 125 percent in 2003 alone.

“The core of our clientele is schools and companies,” Toto spokeswoman Kumi Goto said. “Japanese women are very embarrassed by the sounds they make in a toilet.”

There’s another reason behind the increase in the gadgets: ecology. Women in Japan have traditionally flushed several times to cover up their noises, so the Sound Princess is saving water and cutting down on public building operators’ utility bills.

The Sound Princess is fairly simple. The user passes her hand over a sensor, and the convincing sound of a torrent of water comes from a speaker.

Such gadgets might seem a dainty, modern excess of a shame-obsessed society, but the Sound Princess has deep roots in Japanese culture.

People here are notoriously fastidious: The daily bath is practiced with near-religious fervor, and walking inside with your shoes on is considered filthy. The Japanese word for clean — “kirei” — also means beautiful.

And what happens in a bathroom stall is, well, among the dirtiest things humans do.

Going to the toilet has been considered embarrassing and even shameful for women since long ago, said Noriji Suzuki, a parasitologist at Kochi University Medical School.

“Sometimes you see people talking to each other over a stall in the West, but that would never happen in our culture,” he said.

The trend is not limited to women these days. Some schools have done away with urinals because boys are increasingly too embarrassed to use them.

Tadafumi Morioka, a spokesman for another toilet maker, Inax Corp., said his company started selling a product similar to the Sound Princess in 1988 amid concerns of wasted water.

He said the installment rate of such devices in modern skyscrapers is 100 percent. Inax’s sales increased 25 percent in 2003, Morioka said, although he refused to give precise numbers.

“Most of the demands for the device come from public facility owners and managers, including department stores and elementary and junior high schools,” he said.

But for now, Oto-Hime seems to likely to remain for women only.

“I still haven’t heard of men who say they want Oto-Hime in men’s rooms,” Goto said.