PURPOSE: To presume wave distortion of plate material after cooling during heating with good accuracy by correcting the thermal expansion on a measured result of a hot wave-distortional shape based on the measured result of a temperature distribution of the hot plate material.

CONSTITUTION: When a hot-rolled steel plate 10 is being rolled or after the steel plate is rolled, the width and the edge position are measured by a width edge meter 16 and edge position meters 18 and 20 while the steel plate is conveyed by a table roller 12. Further, at the same time, the temperature distribution of the steel plate 10 in the width direction is measured by a radiation thermometer 22 and the wave- distortional shape of the steel plate 10 is measured by a range finder 24. In this way, the width and the position of the edge at a certain position in the (y) direction and the temperature and height of each position in the (x) direction of the position in the (y) direction are measured and inputted to an arithmetic indicator 26 by a pulse from a pulse transmitter 14 for every prescribed conveyance distance and a map of the width, the edge position, the temperature and the height is prepared. Then, the indicator 26 calculates the flat shape of the plate after the plate is cooled and corrected to the flatness using the distribution of the temperature and height and adding the thermal dimensional correction and the wave-distortional correction.