Under the Imperial House Act, the Empress, the Grand Empress Dowager, and the Empress Dowager are addressed by the title Her Imperial Majesty, and all other members of the Imperial family are addressed by the title of His or Her Imperial Highness (Article 23 of the Imperial House Act).
皇室典範上、皇后・太皇太后・皇太后は陛下、それ以外の皇族は殿下の敬称を称することとなっている（皇室典範23条）