PURPOSE: To perform a quick taking out of materials inside a crusher and obtain an easy control of the material level of the crusher by setting to a mill shell air holes and a casing surrounding the shell and having a secondary air source connected to it.
CONSTITUTION: The primary air and materials are supplied into a rough crushing chamber 5 through a supply port 2 to be roughly crushed. Next, the materials are finely crushed in a fine crushing chamber 6. The secondary air coming from a secondary air source 9 flows into the fine crushing chamber 6 through air holes 7 from a casing 8, preventing the staying of fine particles crushed in the fine crushing chamber 6, and quickly moving the fine particles toward a discharge port 3. The air holes 7, for example, above the mill shell 1 are to be closed and those under the mill shell 1 are to be opened by a seal metal 14, thereby the secondary air coming always from air holes 7 underneath to effectively apply on fine particles.