Who is system administration? I didn't setup this user, but I saw this user in the Audit Log.

It's good to see new faces here in the Community, @mongkolsri-mowaa. I'd be happy to shed some light regarding admin right in QuickBooks Online.

There are two types of admin in QBO.  The primary admin is the main user who has access to every part of the account, they can manage all users and other admin tasks. Additionally, they are the person who initially set up the account.  Next is the company admin, they can do everything the primary admin can do, except edit or remove the primary admin's access.

Another company admin possibly invited that user. Let's go to the Manage users page to verify this. Here's how:

1. In your company account, click the gear icon on the upper right corner.
2. Under Your Company, select Manage users.
3. Click the three-dot icon under action column and View user activity of the other company admin to confirm.

Additionally, let's review the user listed. If they are unauthorized, we can delete them.

You can also use these articles for future reference: 

User roles and access rights in QuickBooks Online

Add and manage users in QuickBooks Online

Please send us a comment if you have any other questions or concerns aside from user access. I'm glad to provide you further assistance. Take care always!