A neutron ray detecting system that is configured to detect neutron rays, the system comprising: a scintillator that is configured to generate light when radiant rays enter the scintillator; an optical fiber that is configured to allow the light generated in the scintillator to be transmitted therethrough; a discrimination unit that is configured to receive the light transmitted through the optical fiber, and to discriminate a detection signal relating to the received light from a signal relating to the neutron rays in a case where a wave height of the detection signal exceeds a determination threshold value; and a determination threshold value setting unit that is configured to set the determination threshold value, wherein the determination threshold value setting unit includes a detection signal acquiring unit that is configured to acquire a first detection signal that is the detection signal of the radiant rays that have entered the scintillator via a filter that cuts the neutron rays, and a second detection signal that is the detection signal of the radiant rays that have entered the scintillator without the filter, a wave height distribution acquiring unit that is configured to acquire a neutron ray wave height distribution of only the neutron rays and a gamma ray wave height distribution of only the gamma rays included in the radiant ray, on the basis of a difference between a wave height distribution based on the first detection signal, and a wave height distribution based on the second detection signal, and a determination threshold value adjusting unit that is configured to adjust the determination threshold value on the basis of the neutron ray wave height distribution and the gamma ray wave height distribution. a scintillator that is configured to generate light when radiant rays enter the scintillator; an optical fiber that is configured to allow the light generated in the scintillator to be transmitted therethrough; a discrimination unit that is configured to receive the light transmitted through the optical fiber, and to discriminate a detection signal relating to the received light from a signal relating to the neutron rays in a case where a wave height of the detection signal exceeds a determination threshold value; and a determination threshold value setting unit that is configured to set the determination threshold value, wherein the determination threshold value setting unit includes a detection signal acquiring unit that is configured to acquire a first detection signal that is the detection signal of the radiant rays that have entered the scintillator via a filter that cuts the neutron rays, and a second detection signal that is the detection signal of the radiant rays that have entered the scintillator without the filter, a wave height distribution acquiring unit that is configured to acquire a neutron ray wave height distribution of only the neutron rays and a gamma ray wave height distribution of only the gamma rays included in the radiant ray, on the basis of a difference between a wave height distribution based on the first detection signal, and a wave height distribution based on the second detection signal, and a determination threshold value adjusting unit that is configured to adjust the determination threshold value on the basis of the neutron ray wave height distribution and the gamma ray wave height distribution.