PURPOSE:To raise the quality of molded parts as well as increase the speed of production by a method in which a molten resin and an organic filler as raw materials are crushed and ground by using a screw having large numbers of projections and differently direcred two grooves. CONSTITUTION:Cylinders 1-3 are coaxially connected, and a screw 4 of a specific shape is rotatably inserted into the connected cylinders. In a material- supplying and plasticizing section A, a mixture of a thermoplastic resin and an organic filler which are introduced from a material supply port 11 is heated to raise the temperaure of the raw materials and the resin is plasticized. In a fiber- beating and mixing region M, large numbers of projections 5 and differently directed two groups of grooves 6 are formed. The aperture G formed by the outsides of the projections 5 and the inner surface of the cylinder 2 can be adjusted by moving the screw 4 forwardly or backwardly. In an extruding and weighing section B, the flow of molten resin is stabilized and the molten resin is sent out to the discharge port 12 at the tip end of the cylinder 3.