There is disclosed a vertical mill comprising a rotatable table and rotatable grinding rollers disposed on the upper surface of the table near the outer periphery of the table. Raw material is supplied to the central portion of the table. A given grinding pressure is applied to each grinding roller to crush the raw material between the upper surface of the table and the outer surface of each grinding roller. Rotatable auxiliary rollers are arranged alternately with the grinding rollers on the upper surface of the table near the outer periphery of the table. The layer of the material caught by the grinding rollers are compacted by the outer surfaces of the auxiliary rollers. There is also disclosed a method of operating the vertical mill. The method consists in separating controlling the grinding pressures applied to the grinding rollers according to the loads on the grinding rollers.