We've scanned every flight from Seoul to Tokyo to find the best deals. If you want a cheap flight, it's best to be as flexible as you can on dates. At the moment, the cheapest flight is on 11 / May / 2020. If that day's no good for you, try searching by month to see some alternative dates.
Make sure you use the search options above to search for flights to other airports in Tokyo, not just New Tokyo International Airport. The more options you look at, the better your chance of finding a cheap deal. Just so you know, the flight typically takes 5 hrs 25 mins.
If you're planning on going to Tokyo during this time, you could take a look at other airport options. Or you could try to get a last-minute bargain.