[en] A position to be measured in the radial direction of a spent fuel pellet, for example, a minute region at the central portion thereof is destroyed by using a microdrill mounted to a spent fuel pellet destruction device. In this case, gaseous nuclear fission products accumulated in grain boundary bubbles in the minute region are released out of the pellet. The released gaseous nuclear fission products are analyzed by using a mass spectrometer and the weight of the gaseous nuclear fission products is measured. Next, the spent fuel pellet destruction device is moved to other position in the radial direction to be measured, for example, to the outer circumference of the pellet to conduct measurement in the same manner as described above. The amount of the released gaseous nuclear fission products present in the spent fuel pellet can be measured rapidly and accurately. (I.N.)