After the atomic bomb was dropped on August 6, 1945, “black rain” containing black-colored raindrops fell in the city of Hiroshima and some surrounding areas. Regardless of the color of the raindrops that actually fell, any rain that fell in and around Hiroshima City from the time of the atomic bombing until the evening is called “black rain”. Seventy years later, in 2015, 84 people who experienced the “black rain” filed a class-action lawsuit in the Hiroshima District Court against the city of Hiroshima, Hiroshima Prefecture, and the national government, claiming that they fell under the category of “those who were likely to suffer physical damage from the effects of atomic bomb radiation at the time of the atomic bombing or after the atomic bombing” as stipulated in Article 1, Item 3 of the Atomic Bomb Survivors Relief Law. Since the lawsuit was filed, 22 oral proceedings had been held. On July 29, 2020, the Hiroshima District Court ruled in favor of the 84 plaintiffs in the “black rain” lawsuit, signifying a complete victory for the plaintiffs. This paper examines the health effects of internal radiation exposure for the Hibakusha that led to the plaintiffs’ victory, including the events leading up to the trial.