I found the size in /var/log/journal/ increases quickly.
One of my VPS has only 40GB disk, the journal takes 10GB.
After searching and I found this solution:
I followed the methods and made the problems resolved.
I only needed to edit the file /etc/systemd/journald.conf and put this line in it:
Then restarted the journald daemon:
service systemd-journald restart
After doing so the dir get cleaned automatically.