The gauge has a square C-shaped frame provided in direction perpendicular to pass-line plane, which conveys to-be-measured object so that object is arranged between arms of frame. A detector unit has a main detector, which is disposed on the radiation source disposed on one arm opposite to the C-shaped frame, and a subdetector, which is disposed on the other arm and can be used to detect transmitted radiation and radiation from object scattered around main detector. A calculator, which uses the stored output change caused by the increasing and decreasing of the radiation detected by the main detector and the output change of the radiation components detected by the sbdetector to calculate the corrective factor, calculates thickness from output difference of detectors obtained by selectingand multiplying correction count when object moves to optical axis direction perpendicular to pass-line plane.