Abstract: An apparatus for grinding the crusts of seeds, trunks, branches and leaves of any plant of a family of grasses, particularly rice hulls, includes an outer cylinder having an inner surface which is similar to that of a mortar, and a screw having a spiral screw thread and extending through the cylinder. The cylinder and the screw are rotatable relative to each other. They define therebetween a space having a gradually decreasing inner cross-sectional area in which the material to be ground is progressively compressed and heated, whereby the material has its structure destroyed and produces fine particles. The apparatus has a high degree of efficiency and a high degree of durability. The ground product has only a low content of lignin having an adverse effect on its compatibility with a synthetic high molecular compound or its dispersibility therein and is, therefore, useful as a filler or modifier therefor.