Here are some quick notes on what you need to do to become a 5G station operator in Japan.

The Ministry of Internal Affairs and Communications (総務省) has a Japanese page with general information on 5G licensing that I recommend reading.

Operator license

ローカル５Ｇ及び自営等ＢＷＡの基地局を扱う無線従事者について、一般 的には、電波法施行令（平成 13 年政令第 245 号）第３条に基づき 第三級陸上特殊無線技士 の資格者を適用する。 Source

As the operator, you need to receive a third level special ground wireless technician license (第三級陸上特殊無線技士.)

Station license

ローカル５Ｇを導入する場合、無線局の免許（電波法第４条）の申請が必 要となる。免許申請については、常時受け付け、標準的な処理期間は約１ヶ 月半である。 Source

You need to apply for a station license with the Ministry of Internal Affairs and Communications. Processing your application takes around one and a half months.

Base station

This is the expensive part of the process.

As part of the application process, you need to specify what your usage scenario is and what the technical implementation looks like. You would want the most “turnkey” device possible without paying for a consulting services with expensive recurring fees.

firecell quotes around EUR38,000 for a indoor user private 5G network with recurrent fees. That’s expensive.

innowireless sells 5G small cells called “QUCELL 5G Sub-6” and “QUCELL 5G mmWave” but I couldn’t find any quotes. A 5g mmWave evaluation kit on DigiKey costs around JPY200,000 at the time of writing. You still need to attach a radio through its M.2 interface, though.

Conclusion

The licensing itself seems like the less complicated part. I don’t understand 5G network architecture well enough to analyze what devices you would need for a full private 5G station.