関門海峡
is a major highway on the islands of Honshū and Kyūshū in Japan. It follows the old Sanyōdo westward from the city of Osaka, Osaka Prefecture in the Kansai region to the city of Kitakyūshū in Fukuoka Prefecture, passing through the San'yō region en route. Between Hyōgo Prefecture and Yamaguchi Prefecture it parallels the Sanyō Expressway; it crosses the Kanmon Straits through the Kanmon Roadway Tunnel. Its total length is 533.2 km. At its Osaka terminus, it meets Route 1; at its western terminus, it links with Routes 3 and 10.