PURPOSE: To enable to simply and easily perform the switch of a specification into the other with the change of an objective crop in a second grade grain- treating device.

CONSTITUTION: The second grade grain-treating device of a thresher in which the second grade grains are retreated by the cooperation of fixed teeth with a driven rotator 22 in a second grade grain-returning passage is characterized by forming an opening in the peripheral wall of a treating case 23a, the opening being opened or closed with a cover 24 and used for cleaning the treating case 23a, forming plural soil-removing holes 25 in the cover 24, and further detachably attaching the fixed blades 21 to the cover 24 in a state that the fixed blades 21 are inserted from the soil-removing holes 25 into the treating device.