The Imperial Throne (皇位)
The notion of the Imperial Throne means the throne of Emperor in Japan. The Imperial Throne is synonymous with the word 'Mikurai' that denotes sitting on "Takamikura" (the Emperor's high seat in the ceremony of the sovereign) which also means the Imperial Throne. To succeed to the Imperial Throne from the previous Emperor is called "the succession to the Imperial Throne" or "Koi wo fumu"(stepping up to the Imperial Throne). Hitsugi' was synonymous with the Imperial Throne in the ancient and medieval Japan. Hitsugi indicates a throne or Imperial Throne that is equivalent to a position of King or Emperor in other countries.
The outline
The Constitution of Japan and the Imperial House Act that took effect in 1947 provide that the position of Emperor (the Imperial Throne) is subject to consensus of Japanese citizens, and that, as for the Imperial succession, a son in the male line who belongs to the Imperial lineage (genealogically the direct descendant in the main branch of the family of Emperor Jinmu) should succeed to the throne. It is not permitted that Emperor hands over the Imperial Throne to another person during his life time (the Imperial House Act, Article 4).
What are referred to "the Three Sacred Treasures of the Imperial Family" can be mentioned as the symbols of the Imperial Throne of Japan as they have been since ancient times. To take responsibility for these treasures from the previous Emperor and maintain them was believed to be a proof of the legitimate Emperor. However, we should bear in mind that having the Three Sacred Treasures of the Imperial Family is not always the crucial condition of the succession to the Imperial throne, as there was a case that Emperor Gotoba even without the three treasures succeeded to the throne. By contrast, some of the succession to the Imperial Throne held by the Northern Court were not always legitimate.