PURPOSE: To obtain feed having optimal size and mixture into plastics by grinding vegetable in making the fiber direction lie in the running direction of the grinding wheel.

CONSTITUTION: On a belt grinder 1, bamboo (b) is ground in making the growing direction (longitudinal direction) lie in the running direction (arrow direction in the figure) of a grinder 1. The vegetable cotton obtained through this grinding is ground in order that the fibrin (a) has a certain length. The length of the fibrin (a) is determined by the pressing degree to the ground surface and angle differences or the like between the growing direction and the running direction, and the belt grinder 1 can be replaced by a gear driven type. As grinding means, it is conducted by the outer periphery of a disc grinding wheel 2 and the side surface of the grinding wheel 2 or by the use of odd-shaped grinding wheel 3 or metallic blade 4.