To accurately measure the distribution of gamma-ray quantity in an axial direction by comparing the signals from detectors which are arranged opposite to each other at the same positions in the axial direction and detecting a displacement from the specified position corresponding to bend of used fuel on the axial direction position.

A plurality of detectors arranged in the axial direction of used fuel 2 are calssified into 3-1A to 3-nA and 3-1B to 3-nB, and they are inputted into pultiplexers 4A and 4B. The multiplexers 4A and 4B are changed over in a manner to select the output of detectors on the same position in the axial direction at the same time. Countes 11A and 11B are connected to the output steps of the multiplexer 4A and 4B. The counters 11A and 11B calculates metered values that the gamma detection signal obtained through detection multiplied by measuring time. An averaging part 14 averages the metered values passing through correction computing parts 12A and 12B, and an indictator 15 converts the average values at respective positions in the axial direction to be inputted through the averaging part 14 into the distribution of the gamma ray quantity in the axial direction, and the result is indicated.