It's Gimpo Airport, which had been No. 1 airport of Korea for long time until Incheon Airport was built. Incheon airport has taken almost all the international air routes and Gimpo got smaller than old days.
But if you want to go to Japan, or come from Japan, you can use Gimpo(Korea)-Haneda(Japan) air route. Though Airports of Gimpo and Haneda are smaller than ones of Incheon and Narita, they are near to downtown. And additionally flight time is shorter and fare is cheaper.