A neutron capture therapy system includes a neutron ray generating unit, an irradiated body placing unit on which a patient (irradiated body) is placed, and a gamma ray detecting unit that detects gamma rays emitted from the patient (irradiated body). The gamma ray detecting unit includes an emission part that emits light or electrons as the gamma rays are incident thereon, an amplification part that amplifies and outputs the light or the electrons emitted from the emission part, a first neutron ray shielding part formed of a substance containing 6-lithium, and a second neutron ray shielding part formed of a light element. The first neutron ray shielding part is provided so as to cover at least a surface opposite to an adjacent surface adjacent to the amplification part, among surfaces of the emission part.