PURPOSE: To finely ground waste straws by a rotor and a net member and to readily clean and inspect the interior of a device, by arranging the net member along the outer periphery of the rotor to grind threshed waste straws and making the rear side of the net member rotatable.

CONSTITUTION: Waste straws sent by a waste straw conveyor 11 are cut into 3W5cm at a cutting part 12 and sent to a grinder 13 by a raking-in rotator 22. In order to improve grinding efficiency, the waste straws are finely ground through blows by a grinding rotor 25 having the tip branched into Y shape and through cutting action of edges of meshes of a net member 26 comprising an expanded metal, cropped from the meshes and discharged by a conveyor 14. A rear cover 45b of a waste straw treating device 5 is shakable around a shaft P and since the net member has a rear side movable part 26b shakable and openable in an integrated way with the rear cover, the interior of the grinding device is readily cleaned and the grinding device is easily repaired.