PURPOSE: To reduce the thermal effect on a device by providing a descaling device vertically movably to the upper part of a rolling mill.

CONSTITUTION: A hood 1 of descaling device is connected to a hydraulic cylinder 4 through a pin 10, and the cylinder 4 is fixed to a frame of rolling mill through a cylinder pin 12. When a billet 7 required to be descaled is rolled, the hood 1 is lowered to a prescribed position by stroking the cylinder 4 to descale the billet 7 by the high-pressure water of a nozzle 19. In case of a billet 8 not required to be descaled, the thermal effect of billet 8 is decreased by lifting a rod 14 of cylinder 4. In this way, the descaling device is effectively used and the thermal effect by the billet 8 is decreased.