A neutron detector without 3He gas, provided with a translucent type plate neutron scintillator having the structure capable to emit fluorescence from double-sides; the neutron scintillator is composed of ZnS fluorescent substance and a neutron converter which contains 6Li or 10B, and arranged at an angle of 45 degrees from the neutrons which are incident in parallel all together, inside of a cylindrical detector housing with the circular or square section where the specular reflector with the reflectance of 90% or more is arranged internally, andthe fluorescence emitted when the neutron enters the scintillator is detected by two photo multipliers arranged on both sides, and signals output from these two photo multipliers are processed to be taken out as a neutron signal.