[en] Purpose: To simply and easily measure the neutron multiplying factor, total amount of nuclear fissile materials and burnup degree in a non-destructive manner. Constitution: A neutron detector is disposed on one side and an external neutron source is disposed on the opposite side of a nuclear fuel assembly in a water system. The detector is covered with lead or like other shielding materials so as to decay gamma-rays, of neutrons and gamma-rays, issued from the assembly thereby measuring the neutrons at high accuracy, while the neutron source is contained within a container having a shutter device using neutron absorbers such as cadmium. Displacement for the measuring position of the assembly is carried out by an instruction from a mathematical processing section by way of a fuel assembly driving device and a control section and the signal from the detector is inputted to the mathematical processing section by way of a neutron measuring section. (Sekiya, K.)