Is it illegal to use a device that interferes with mobile radio waves?

Is it illegal to use a device that interferes with mobile radio waves?

With the proliferation of cell phones, annoying phone calls that ignore etiquette are also evident. There seem to be more and more cases of such calls being disrupted using commercially available 4G phone jammers. This device, if cheap, can be bought for less than 2,000 yen. On the other hand, is it illegal to interfere with calls? but? A coffee shop that can't use mobile phones!

All work communication can be done with a mobile phone. Mr. M carries two mobile phones with him, a mobile phone and a powerful underground PHS, so that he can answer the phone at any time, no matter where he is in the train, coffee shop, etc. Mr. M's habit is, "What is the work value of an incoming call?"

One day, when I took out my cell phone to make a call at a coffee shop near Shinjuku, I found that it said it was out of service. Even if I take out the PHS, it doesn't work. Even if you rush out of the coffee shop, it still won't work. After about 3 meters from the entrance, it was finally possible to make a phone call.

10 Antenna Cell Phone Signal Jammer

However, there are no obvious obstacles around this coffee shop, nor is it a difficult environment for radio waves to reach. Mr. M is surprised that even indoors, such as underground or department stores, radio waves cannot reach places with good visibility.

In fact, at this coffee shop, there are multiple wifi jammer placed in the store that interfere with the radio waves of commercially available mobile phones.