PURPOSE:To contrive to ease detection, by rotating and driving a boundary sensor to detect a boundary by a pulse motor while reciprocating and rotating in left and right direction and by determining a boundary position detected by the boundary sensor by a pulse signal to drive the pulse motor. CONSTITUTION:When a signal of normal rotation or reverse rotation is started from a control device 4, a drive pulse signal is started from a pulse generating pulse 5 and a pulse motor 2 is revolved in a number of rotations proportional to the number of the drive pulses. A signal is outputted from the control device 4 to a boundary sensor 1, detected information from the sensor 1 is inputted to the control device 4 and outputted to a display device 6. The boundary sensor 1 starts ultrasonic wave, receives the reflected wave, detects an obstacle, detects reflected wave signal from grain straw while separating from the reflected wave from cut field side so that the boundary between the cut field and the uncut field is detected.
