...
deb http://ftp.jp.debian.org/debian/ stretch main non-free
deb-src http://ftp.jp.debian.org/debian/ stretch main non-free

deb http://security.debian.org/debian-security stretch/updates main non-free
deb-src http://security.debian.org/debian-security stretch/updates main non-free

# stretch-updates, previously known as 'volatile'
deb http://ftp.jp.debian.org/debian/ stretch-updates main non-free
deb-src http://ftp.jp.debian.org/debian/ stretch-updates main non-free