Hi @su1sin, welcome to our forum! When you're in Japan your phone will choose the strongest network. Which one that is can differ per location. If you want to use a specific provider you can set your phone to that provider, so it'll only connect to that. A few years ago I was in Japan too, in the 3 weeks I was there I've seen several local providers. 

It's also possible to get a local Japanese simcard. If I recall correctly there is a shop on Narita that provides those, very close to the JR (Japan Rail) ticket office. If you have one of those you'll be connected to the network you choose.

Hi Jasper, thanks to you, my question has been resolved! I highly appreciate your great support:)