PURPOSE: To seize a plate thickness profile while rolling and to control dynamically the flatness, the shape, and the thickness of a plate material by providing a three-head type gamma-ray thickness gauge to the front side or the rear one of a finish mill and installing further an one-head type thickness gauge additinally.

CONSTITUTION: An one-head type gamma thickness gauge 8 and a three-head type one 9 are adjacently arranged in the longitudinal direction of a rolling line at the front side or the rear side of a finish mill 1. While rolling, the plate thickness tA at the center position in the plate width direction is measured by a head 3 of the gauge 8, and the plate thicknesses tB, tC, tD at otional three positions B, C, D are measured by heads 4, 5, 6 of the gauge 9 respectively. A plate thickness profile and a plate crown in the plate width direction are obtained respectively basing on the measured results. The detected results are provided to a rolling reduction device and a roll bending device, etc. to control dynamically the flatness,the shape, and the thickness, etc. of a plate material while rolling.