The "Sea of Japan" starting with the Korean peninsula, is bordering Japan and Russia, etc. It is not just Japanese sea but the sea that many countries share. Korean call that sea "East sea", but Japanese are calling 'Sea of Japan'. The term 'Sea of Japan' is deeply related to the history of the International Hydrographic Organization. In order to have standard international titles of the sea and ensure safe voyage, the International Hydrographic Organization establishes these international standards.
In 1929, the IHO published a book titled ¡°Limits of Oceans and Seas¡± in which the names of the ocean were internationally standardized. The 'Sea of Japan' began widely used internationally from the release in 1929. Korea was under Japanese occupation and thus could not participate in publication. Therefore, it unilaterally reflects Japanese position in the international community. However, the Korean name 'East Sea' has been the name they have used for over 2,000 years not only in Korea but on world maps and even on old Japanese maps.
The IHO and the United Nations Group of Experts on Geographical Names or UNCSGN, the international organizations which determined the standard names of the seas in the world, suggests that in the hard case on the agreement of a name for a landmark shared by 2 countries, it is usually recommended to use both names of two countries.
Therefore, It is a mistake to refer to the sea as the "Sea of Japan." I suggest you write "East Sea" where you marked "Sea of Japan." on the map.
Thank you & Hope your any reply soon,
Elin Lee