Fukuoka airport is an international airport located in Fukuoka, Kyushu island. It is one of the busiest airports in Japan and it is the busiest Japanese airport which has single runway. It is also listed as level 3 by Worldwide Slot Guidelines of IATA.
Finnair operates seasonal flights between Helsinki and Fukuoka, which is the only international flight between Fukuoka and Europe. The busiest domestic route is Fukuoka - Haneda which 42835 flights flew in 2017, which made Fukuoka-Haneda as the 4th busiest domestic routes by frequency in the world. By the way, Sapporo - Haneda is the 6th busiest domestic route. Nations which has multiple Top 10 busiest domestic routes are only Japan and Australia. It is interesting that both nations do not have Top 20 busiest international routes at all. On the other hand, frequency can be low but the total number of passengers per route can be higher.
Fukuoka airport is known as the closest airport from downtown in Japan. It is just 5 km away from downtown Fukuoka, just two subway stations. Then, it is the only aiport in Japan which is connected by subway. Although Haneda and other major airports in Japan are connected with downtown by rail but by overground trains only.