PURPOSE: To precisely measure the thickness of a crad layer deposited on the bottom of a water tank in facilities to treat radioactive substances even in a condition such as muddy water or the deep depth of water by detecting radioactivity generated from the crad.

CONSTITUTION: A closed type cylinder 3 provided with a cylindrical support 9 and sharped at its point is inserted into the bottom of a water tank, a radioactivity detector 5 covered with a shielding case 6 having a GM tube survey meter sensor 7 in its inside and a slit 8 at the side is hung with a graduated rod 4 and inserted down to the bottom of the cylinder 3 and then the division of the graduated rod 4 is set to a zero point at the part contacted with the upper end of the cylinder 3. While reading out a survey meter 10, the graduated rod 4 is pulled up and the division of the graduated rod 4 contacted with the upper end of the cylinder 3 is read out at the time when the reading of the survey meter 10 is turned to an extremely small value. The sum of the distance between the zero point and the read-out division and the distance between the slit 8 and the bottom is the thickness of the deposited crad layer.