Hi @Yuki Maeda, Celyn from the Figma Community Support team here.
Happy to help! I can see that you're on a Professional plan, and just wanted to clarify that you won't see a Transfer owner option because there isn't a separate owner role on this plan.
If you'd like another member to take over as an admin, you can grant them admin access by following these steps:
From the file browser, select All folders from the left sidebar.
On the team page, click the down arrow next to the team name and select View members.
Find the relevant member, click their existing permissions dropdown, and select Grant admin access.