PURPOSE: To enable measurement of corrosion in short time and efficiently by providing a positioning mechanism to hold the two diagonal corners of a fuel channel from both sides which is set on a cylindrical frame, and a sensor detectable of surface corrosion in a fuel channel.

CONSTITUTION: A fuel channel is firstly drawn up with a refueling machine and inserted in the frame 6 of corrosion measuring device. The measurement of corrosion is set at ten points along the fuel channel axis, and the positioning of them is done by up and down of the fuel channel with the refueling machine. The position of the fuel channel is measured by the drawn length of the rope hanging it. After inserting the fuel channel into a proper position, an air cylinder 13 extends to have arm 12 approached to a corner guide roller 11. The fuel channel is held at two diagonal corners by the roller 11 and the arm 12 from both sides and rotational positioning of the fuel channel is done by a positioning mechanism 10. Then an air cylinder 15 extends and each contact type sensor 18 approaches to the fuel channel and contacts to its surface.