PURPOSE: To make it possible to easily measure the undulating state of the surface of a material to be measured by suspending the material to be measured down into a radiation shielding liquid and passing a sensor therethrough.

CONSTITUTION: In measuring the dimension of a fuel channel 3, a fuel assembly 1 is hung by means of a hanging tool 21, and passed through a radiation shielding liquid 8 to shift the fuel assembly on a sensor 11 installed in a fuel storage pool. A signal corresponding to the vertical position of the hanging tool 21 upon hanging down the hanging tool 21 and passing the fuel assembly 1 on the sensor 11 and a signal corresponding to the undulation on the surface of the fuel assembly 1 from the sensor 11 are led to a memory device 41 through cables 31 and 32. The hanging tool 21 is hung down until the upper part of the fuel assembly 1 reaches the measuring head of the sensor 11, and the signal is continuously recorded. An operator 42 delivers as output the deformation state of the fuel assembly 1 over its full length by the signal from the memory device 41.