Japan has the oldest unamended constitution in the world. We examine the determinants of its longevity and recent proposals seeking its amendment. While traditional revision attempts have focused on ideological provisions such as Article 9 and the imperial system, changes to political institutions, particularly bicameralism and local governance, have become a focal point since 2005. Our comparative analysis demonstrates that the constitution’s enumeration of institutions is uncommonly vague, leaving room for significant reforms. However, we also show that the most recent (2012) amendment proposal by the Liberal Democratic Party (LDP) would not “fix” this lacuna, leaving the constitution an international outlier.