PURPOSE: To eliminate excessive speed change and to lessen hunting, by sending and controlling various speed change output signals regulated and outputted by sending of pulse signals by a load sensor in a state of a specific time lag.

CONSTITUTION: Various speed change output signals controlled and outputted by sending of pulse signals by detection of load change by a load sensor 2 are sent in a delayed state by a corresponding time required for transfer and supply of reaped grain straws of a reaping device 3 to a thresher 4 by a delaying device. Speed change is mildly carried out and speed change response is likely to coincident with actual load.