In the first chapter of the former Imperial Family Law, it says 'the Imperial Throne of the Empire of Japan should be succeeded by male Imperial Family members who are in the line of the successive Emperor.
皇室典範第1条では「大日本國皇位ハ祖宗ノ皇統ニシテ男系ノ男子之ヲ繼承ス」と記されている。