To eliminate the interference of a cutter cover with a waste straw transfer chain in the case of cutting waste straw by erecting the cutter cover and to provide a protection plate effective for preventing the deposition of thrust grain discharged in a state thrust into straw dust or waste straw and smoothing the discharge of waste straw in discharging the straw backward in uncut state while protecting the cutter with the protection plate.

A cutter cover 5 tiltable around a supporting point P above a rear-side cutting disk blade 4 is attached to a disk cutter C for cutting waste straw. The cover 5 is switchable between a state to cut the waste straw and a state to discharge the straw without cutting. The upper circumference of the front-side disk blade 3 is covered with a protection plate 6 freely movable forward to enable the switching between a protecting posture and a non- protecting posture. The change of the protection plate 6 between the protecting posture and the non-protecting posture is preferably interlocked with the lifting and lowering motions of the cutter cover 5.