please take a look at these maps.
http://www.freeworldmaps.net/asia/southkorea/map.html
http://www.freeworldmaps.net/asia/japan/index.html
One is map of korea and the other is map of japan.
Though it is same sea, one is written as "East sea" and the other is written
as "Sea of japan".
during the 19th century, as Japan's power got bigger, Foreign people began to
call "East sea" to "sea of Japan".
But this is wrong. There was no reason to call it "sea of japan".