A control apparatus, particularly for roller mills in plants for milling food products, including a probe device which is inserted in a container of a product to be fed to a pair of milling rollers by feed rollers. The device includes a reference probe (2), which is always immersed in the product and measures a reference capacitance value, and a level probe (20), which measures a variation of the capacitance of the product related to the variation of the level of the product. The reference capacitance value and the capacitance variation are used by a control means to actuate the rollers. The probe device is pre-calibrated in factory so as to be able to adapt automatically to the variations of the characteristics of the product to be fed to the rollers.