PROBLEM TO BE SOLVED: To properly determine a range of cooling a material to be rolled which is transported, even if a shape of a front end or a tail end of the material to be rolled after being rolled is not rectangular.SOLUTION: A table for storing a front end shape of a thick plate, a target plate width of the thick plate, a target plate thickness of the thick plate and an operation position specifying length in association with each other is created beforehand. After that, an image of the whole front end of the thick plate is taken, the shape of the front end of the thick plate is determined based on the taken image, and the operation position specifying length corresponding to the determined shape and the target plate width/target plate thickness of the thick plate is read from the table. Then, the supply of a cooling water by a cooling spray is made to start, when a position spaced apart to the side of the tail end from a current plate width-direction center position of the front end of the thick plate
