As I expected, a new user on instance which has multiple admins auto-follow all admins on instance as default behavior.
It may be a problem on instance which has many admins (for report solving?).
It seems better creating announcement only account for auto-follow?
新規アカウント作成時のauto-followはadminアカウント全部っぽいので告知専用アカウントを作ってそれを設定した方が良いのかも知れない。