Contrary to its humanist image, Hidankyo, the Japan Confederation of Atomic and Hydrogen Bomb Sufferers Organizations, has engaged in contentious politics against the state for decades. This article traces the little-known history of the Hidankyo movement from the mid-1950s to the early 1980s, introducing how this organization formed in relation to the movement to ban nuclear bombs in the mid-1950s and how it grew into an independent social movement organization with clear policy demands after overcoming an organizational crisis triggered by the Cold War politics of the 1960s. The movement slogan for Hidankyo, “no more hibakusha,” did not naturally emerge from the sufferers’ experience with the atomic bombings, but was substantiated through their struggles to confront their adversaries, most importantly the Japanese government.