After the Fukushima Nuclear Power Plant accident in March 2011, a large-scale ultrasound examination of childhood thyroid cancer for all Fukushima residents aged 18 years old or younger was initiated. Fukushima was divided into four areas according to the external radioactivity released by the accident: the highest (A), high-intermediate (B), low-intermediate (C), and the lowest (D). Five rounds of surveys were carried out from October 2011 to March 2023. The annual incidence rates of the "Common Case" not affected by the accident were able to be estimated. The difference between the incidence rate of whole patients and the "Common Case" is that of the "Radiation-induced Case". The annual incidence rate of the "Radiation-induced Case" began to increase immediately after the accident, where the highest level was seen in A area, and the order was A > B > C > D. It showed that the development of childhood thyroid cancer was affected by the radiation released by the accident. The effect of the radiation consisted of two phases: the first phase may have been due to the damage to the immune system, and the second phase may have been due to the genetic mutation in the children who were youngest at the time of the accident.