This is done through the MNP system (Mobile Number Portability). 

About MNP:
You will have to request an MNP number from your current provider (online or in-person). This number will let you keep your current phone number even if you move to another provider.

MNP numbers, once issued, are valid for 14 days.

We need the MNP number to have more than 8 days of validity when you sign up with us.

Useful links:

Request MNP number from Docomo

Request MNP number from Au (In Japanese only)

Request MNP number from Softbank (In Japanese only)

The sign-up process would be as follows:

1. Request an MNP number from your current provider

2. Go to our Sign-up page

3. Click on the checkbox below

Note: This MNP option is available for SIM cards only. For eSIM, check this FAQ article: MNP Port-in for eSIM

4. Provide the required information

4. Submit and complete the rest of the application

We will follow up regarding the MNP activation schedule in an email once your application is authorized. Please check your inbox regularly and read our emails carefully for more details.