Provided is a neutron capture therapy system and a gamma ray detector for neutron capture therapy capable of detecting gamma rays with high accuracy even in a high-intensity neutron field. A neutron capture therapy system (1) includes a neutron ray generating unit (10), an irradiated body placing unit (20) on which a patient (H) (irradiated body) is placed, and a gamma ray detecting unit (30) that detects gamma rays (G) emitted from the patient (H) (irradiated body) . The gamma ray detecting unit (30) includes an emission part (31) that emits light or electrons as the gamma rays (G) are incident thereon, an amplification part (32) that amplifies and outputs the light or the electrons emitted from the emission part (31), a first neutron ray shielding part (33) formed of a substance containing 6-lithium, and a second neutron ray shielding part (34) formed of a light element. The first neutron ray shielding part (33) is provided so as to cover at least a surface opposite to an adjacent surface (31a) adjacent to the amplification part (32), among surfaces of the emission part (31).
