PURPOSE: To perform the quantitative measurement of a warping angle of a rolled steel plate, by providing a radiation source and a detector at the output side of a rolling mill to detect the transmitted radiant rays and then detecting both quantity of non-transmitted radiant rays and that of transmitted radiant rays.

CONSTITUTION: The radiant rays delivered from a radiation source 6 or γ rays or X rays provided at the output side of a rolling mill are made incident to radiation detector 7 after transmitting through a rolled steel plate 4. The output of the detector 7 is fed to an operator 8 and compared with the intensity of radiant rays that is obtained with the absence of the plate 4. Then the apparent thickness of the plate 4 is calculated and compared with the actual thickness to calculate the warping angle of the steel plate 4. This angle value is transmitted to a plate warp preventing mechanism 9.