33°57′57.4″N 130°57′47.2″E / 33.965944°N 130.963111°E / 33.965944; 130.963111 The Shin-Kanmon Tunnel (新関門トンネル, shin-kanmon tonneru) is a 18.713-kilometre (11.628 mi) long undersea railway tunnel under the Kanmon Straits connecting Shin-Shimonoseki Station and Kokura Station.

At the time of opening in March of 1975, it was the longest railway tunnel in Japan before being overtaken by in 1988 the opening of the Seikan Tunnel. It is also the longest tunnel section on the entire Sanyo/Tokaido Shinkansen line.

• Starting point: Ichinomiya Gakuencho, Shimonoseki City, Yamaguchi Prefecture
• End point: 2-chome, Shimotomino, Kokurakita-ku, Kitakyūshū, Fukuoka
• Overall length: 18,713 metres (11.628 miles)
• Undersea bed section: 880 metres (960 yd)
• Below sea level section: 66 metres (72 yd)
• Tunnel gradient: 18/1000
• Track gauge: 1,435 mm
• Electrification: 25 kV AC, 60 Hz, overhead catenary

The construction of the new tunnel started as a project to connect Honshu and Kyushu with a high-speed railway. The tunnel was constructed at the shortest section in the Kanmon Strait between Dannoura, Shimonoseki City - Mekari, Mojiku, Kitakyushu. Although the entire Shin-Kanmon Tunnel is 5 times longer, the undersea section is shorter than the conventional line Kanmon Tunnel (the Kanmon Tunnel undersea bed section is 1,140-metre (1,250 yd) long).

There is a 1,000 km (620-mile) distance marker about 1.8 km (1.1 miles) from the middle point of the tunnel. This is the only 1000 km railway marker in Japan.

• Shimonoseki entrance: 34°00′11″N 130°56′53″E / 34.003093°N 130.948167°E / 34.003093; 130.948167
• Moji entrance: 33°52′58″N 130°53′55″E / 33.882837°N 130.8985°E / 33.882837; 130.8985