PURPOSE: To display the gap between fuel rods at the outer part in two dimensions, by detecting the response times from the fuel rods and the height of echo by an ultrasonic wave sensor.

CONSTITUTION: A fuel assembly 1 is suspended from a suspending jig 2 and arranged at a specified position in the vicinity of a sensor device 4 under this state. The sensor 4 is composed of a driving motor part 5, a carrier 6, an ultrasonic wave probe sensor 7, and a cable 8, and attached to a supporting table 3. The response time period from the time when the ultrasonic wave, which is transmitted from the sensor 4, is reflected by the fuel rod 1 to the time when the wave is received by the sensor depends on the distance between the ultrasonic wave sensor to the fuel rod, which is an object. The sensor is moved at a constant speed and performs the transmission and reception. The sensor detects the response time of the ultrasonic wave from each fuel rod, and detects the height of echo and the position of the sensor from the fuel rod. Based on the measured value of the displacement and the gap, the positions of the fuel rods can be displayed in two dimensions.