Abstract: An apparatus for descaling semi-finished products, such as thin slabs or steel strips in rolling mills by highly pressurized water that is sprayed through a nozzle device onto the surface of the semi-finished product is characterized in that the nozzle device is movable with a vertical component above the surface of the semi-finished product by means of a drive. The drive can be controlled by a control device that receives a target distance as input value and an actual distance between the nozzle device and the surface of the semi-finished product detected by means of a distance sensor. The control device supplies an output signal via a signal line to the drive for resetting any deviation to zero. Hence, the nozzle device is always kept at a desired target distance even if humps such as bulgings occur on the surface of the semi-finished product.