PURPOSE: To enable the prevention of improper cutting of a part of waste straw and surely prevent the damage of rotary blades caused by the hitting of hard foreign bodies, when a fixed blade is shifted to a non-cutting position.

CONSTITUTION: In a combine, a chopper A consisting of rotary blades 13 and a fixed blade 14 is placed on a straw-discharging path starting from the finishing end of a treating section and the fixed blade 14 is made shiftable between a cutting position and a non-cutting position. The combine is provided with a speed-changing apparatus X which changes the driving speed of the rotary blades 13 between a high speed state and a low speed state. It is also provided with interlocking operation means 26, 27 and 28 which interlock between the fixed blade 14 and the speed-changing apparatus X in such a manner that the rotary blades 13 are shifted to the high speed state when the fixed blade 14 is at the cutting position and to the low speed state when the fixed blade 14 is at the non-cutting position.