Companies in the industry say the blame lies with people who misuse these applications rather than the programs themselves.
According to Japan's National Police Agency, 1,741 cases of illicit photography were reported in Japan last year, a 1.6-fold increase from 2006.
On Nov. 12, a man was arrested at a train station in Kawaguchi, Japan, after he took photos up the skirt of a female vocational school student with his smartphone as she stood on an escalator.
The man reportedly told police he used an app that silenced the shutter sound to prevent his target from noticing what he was doing.
A man arrested in September after he photographed a woman's underwear in Tokyo also reportedly told police he had used such an app to stealthily take photos about 20 times.
The largest number of snap-happy voyeurs was reported in Kanagawa Prefecture. "About 30 percent of cases involved the misuse of smartphone apps," a senior Kanagawa prefectural police investigator said.
The latest applications include "upgraded versions" that enable people to silently take photos while an e-mail or website is displayed on the phone's screen to provide cover for the surreptitious picture-taking.
"We can't help but think these apps are designed specifically for taking sneaky photos," another senior prefectural police investigator said.
The shutter sound emitted when a regular cell phone takes a photo is voluntarily installed by phone companies to deter users from taking photos without a subject's knowledge. It cannot be disabled.
But the situation differs for smartphones.
According to major cell phone carriers NTT Docomo Inc., KDDI Corp. and SoftBank Mobile Corp., smartphone cameras come equipped with a shutter sound. But one main feature of smartphones is that users can customize the settings _ including adjusting or neutralizing the shutter sound, according to the firms.
A search for the Japanese words "muon" (silence)" and "kamera" (camera) on app sites for Apple Inc. and Google Inc. smartphones turned up about 200 applications. Some boasted they enabled users to "take photos in silence without bothering others," and others said the function "was perfect for taking photos undetected." Some of these programs have been near the top of app ranking charts.
Apple Inc. developed the iPhone, and Google Inc. created the Android operating system.
A representative of Apple Japan defended the availability of the apps.
"There's no problem as long as the developer's stated purpose for the app doesn't go against social ethics," he said.
A Google Japan spokesman said: "A market is a place where developers respond to users' needs. It's up to users to follow etiquette when they use the apps."
The two companies do not plan to remove these apps from their sites.
Japan's Internal Affairs and Communications Ministry said it does not have the legal authority to regulate these apps or mobilize government offices to issue administrative guidance.
"Application markets aren't covered by the Telecommunications Business Law," an official of the ministry's information security section said.
But Keio University Prof. Keiji Takeda, an expert on information security, said some rules were needed for these apps.
"There are limits to legally regulating smartphones whose settings can easily be changed," Takeda said. "However, from a corporate ethics viewpoint, we shouldn't ignore the fact that they're being misused for crimes. We need to consider guidelines for screening and putting apps on the market."