PURPOSE: To smoothly grind the granular substance without break by allowing the granular substance thrown into the space between inner and outer rotating bodies to repeatedly contact with grinding surfaces of both a rotating bodies.

CONSTITUTION: In the rotating body 2 having the grinding surface 1 on its inside and the rotating body 4 having a spiral grinding surface 1 on its outside are arranged to form the space 3 between them. With the rotating bodies 2 and 4 rotated at a given speed, humid crude sand or the like is thrown into the space 3 from a port 5 and allowed to contact with both the grinding surfaces 1 and 1 of the rotating bodies 2 and 4 in order to grind the surfaces of the sand by rubbing and cutting.