PURPOSE: To measure the thickness of an oxide film on a contact part nondestructively in the water of a fuel storing pool by bringing a measuring head for measuring the distance between the head and a metallic body into contact with an optional position on the surface of a fuel channel by using eddy current.

CONSTITUTION: A measuring head 10 for measuring the distance between the head and a metallic body by using eddy current is moved in the horizontal direction by an X-Y motor-driven table 8 while being fixed on a supporting rod 9 and a fuel channel 5 vertically set up in the water in a fuel storing pool 1 is moved in the vertical direction by a preparation machine 3. Consequently the measuring head 10 is brought into contact with an optional position on the surface of the fuel channel 5 to measure the thickness of an oxide film applied to the contact part.