Take the “manhole bag,” a purse that can hide valuables by unfolding to look like a sewer cover. Lay it on the street with your wallet inside, and unwitting thieves are supposed to walk right by. There is also a line of knife-proof high school uniforms made with the same material as Kevlar, and a book with tips on how to dress even the nerdiest children like “pseudohoodlums” to fend off schoolyard bullies.

There are pastel-colored cellphones for children that parents can track, and a chip for backpacks that signals when children enter and leave school.

The devices’ creators admit that some of their ideas may seem far-fetched, especially to crime-hardened Americans. And even some Japanese find some of them a tad naïve, possibly reflecting the nation’s relative lack of experience with actual street crime. Despite media attention on a few sensational cases, the rate of violent crime remains just one-seventh of America’s.

But the devices’ creators also argue that Japan’s ideas about crime prevention are a product of deeper cultural differences. While Americans want to protect themselves from criminals, or even strike back, the creators say many Japanese favor camouflage and deception, reflecting a culture that abhors self-assertion, even in self-defense.

“It is just easier for Japanese to hide,” Ms. Tsukioka said. “Making a scene would be too embarrassing.” She said her vending machine disguise was inspired by a trick used by the ancient ninja, who cloaked themselves in black blankets at night.