This is the method I use when traveling to Vietnam.

Many people are probably familiar with it, as it’s been featured on YouTube and the web.

Currently, some services allow you to use Japanese SIM cards as is, but this method is especially convenient for those staying 15 days or longer, frequent travelers, or those traveling to multiple countries.

The following are the websites I referenced:
Air Sim Official Website: https://www.airsim.com.hk/

Air Sim Japan Authorized Distributor: https://airsim-hk.jp/

Mobi Fone: https://www.mobifone.vn/

This is a very general explanation, so it might be difficult to understand.

If you have any questions, please let me know in the comments section.

I will answer to the best of my ability.