@Ichihashi kazuki - If you are using a paid plan, please contact the Miro support team:

https://help.miro.com/hc/en-us/articles/360020185799-How-to-contact-Miro-Support

If you are using a Free Plan, you are better off to have someone on your team create a new Miro Free Plan team using any email address they have never used with Miro, then have them invite you all to that new team (including their current email address) - now you are all members on both teams (except for the old admin).

Next, you open all boards from your old team and duplicate them I to the new team.

Finally, you can all leave the old team. Now you have anew team with all of your boards in that new team - you ahod also have the Team Admin of your new team promote someone else to the role of Team Admin.