An upper detector and a lower detector that face at least one side of a fuel assembly, on which neutrons are irradiated in a nuclear reactor, and detect radiation are set at a predetermined interval in an axial direction of the fuel assembly. Distributions of radiation signals are measured by the upper detector and the lower detector while the fuel assembly and the upper detector and the lower detectors are relatively moved along the axial direction of the fuel assembly. Soundness of radiation signals measured by the upper detector and the lower detector is determined in every measurement by comparing radiation signal distributions obtained by measuring the same portion in the axial direction of the fuel assembly in a multiplexed manner with the upper detector and the lower detector. Thereafter, relative burn-up is calculated by utilizing the measured radiation signals to measure a burn-up profile. According to the present invention, it is possible to measure a burn-up profile of the fuel assembly while securing reliability of a measurement result.