PURPOSE: To effectively perform the grinding and surface modification of an object to be processed by cutting the object to be processed supplied to a casing by a rotor having blade wheel stages to apply impact action thereto by the rotor and the casing.

CONSTITUTION: The granular object to be processed supplied to a cylindrical casing 1 from one end thereof is processed by the casing 1 and the rotor 2 rotationally driven within the casing 1 while guided to the other end of the casing 1. Many blade wheel stages 22 wherein a large number of blades 22b are arranged at predetermined intervals in the peripheral direction at a predetermined twist angle α so as to cross a matrix line L are arranged to the rotor 2. As a result, the grinding and surface modification of the granular object to be processed can be simultaneously and homogenously performed.