Abstract: A screw mill, for example, used for producing fine powder grains of calcium carbonate used as a bulking agent for paper. According to the screw mill of the present invention, the combination of the continuous screw and discontinuous vanes makes it possible to efficiently grind both slurry and dry materials without causing clogging of the mill. Also the clearance between the hub and the rotor can be finely adjusted by the screw type adjustor. This makes it possible to easily adjust the grain size and to produce ground materials having the spheroidicity of 0.3-0.03 which is best suited for use as a bulking agent for paper.