Dear @宇田川 俊介,
Good day. Thank you for posting your question in the Microsoft Q&A forum!
First of all, I’m sorry you’re having a problem and for the situation you encountered, I deeply understand the inconvenience caused and apologize for it.
Based on the behavior you described, this inactive user entry in the Microsoft Business Center is related to Volume Licensing Service Center rather than standard Microsoft 365 user management.
To ensure you receive the most accurate assistance and that the inactive account is handled correctly, I recommend contacting Microsoft Volume Licensing Support directly. The VLSC support team has the appropriate tools and permissions to review and assist with account management issues related to volume licensing.
You can reach them using the official Microsoft link below: Contact volume licensing support
Please understand that as forum moderator, my primary goal is to provide helpful guidance and support through general troubleshooting steps. While I don’t have access to internal systems required to resolve account issues, I truly appreciate your understanding of these limitations. I genuinely hope the information I share helps guide you in the right direction, and I'm always here to assist as much as I can within my scope.
Please feel free to reply below if you have any update or further concern. Thank you for your valuable time and cooperation. I am looking forward to your response!