After the Meiji period, when the emperor of Japan became the head of state, he began to use the title of 'the emperor of Japan' or 'Nihonkoku Tenno' in foreign affairs as well, and the title of 'the king of Japan' fell into obscurity.
明治時代以降は天皇が最高権威者となり、対外的にも「日本国皇帝」もしくは「日本国天皇」の号を用いたために「日本国王」号は用いられることが無くなった。