PURPOSE: To perform a smooth and accurate control by considering not only the height from the ground but the degree of its deviation when detecting the height from the ground using an ultrasonic sensor 7 and controlling the elevation of a unit of a combine for pretreatment before reaping.

CONSTITUTION: A signal sent from a transmitter 7a of an ultrasonic sensor 7, reflected by the ground and received by a receiver 7b is inputted into CPU 10 and the deviation of the detected reaping height value from the preset reaping height value is calculated there. The single difference value of the deviation is also calculated and the directed amount of operation is determined by using the deviation value and the resultant single difference value as variable input data and using a fuzzy inference role table for output of the directed amount of operation. According to the determined value of the directed amount of operation, the elevation operation of a unit for pretreatment before reaping is controlled.