PURPOSE: To produce a powder efficiently even from an org. natural polymer by contracting the inlet opening part of an inlet gap part from a specific max. dimension to a min. dimension and connecting a grinding gap part to the contracted part and specifying a shearing speed.

CONSTITUTION: In an apparatus equipped with inlet gap parts 5, 25 and grinding plastic with a min. density of at least 0.005 g/cm3 or a plastic-containing compsn., compd. or mixture, one side of the inlet gap part 5 is formed by the jacket surface 6 of a roller 1 and the other side thereof is formed by the recessed opposed surface 7 of an opposed block 2. The inlet gap part 5 is contracted from the max. dimension of 100 mm of the inlet opening part thereof to the min. dimension of 0.025 mm thereof and an inherent grinding gap part 8 is connected to this contracted part and a shearing speed is adjusted to 100-20,000 sec-1 herein. By this constitution, since only one roller is used, this novel roller apparatus is structurally simple and reduced in trouble.