PURPOSE: To reduce the measurement error, avoiding the increase of cost, by arranging a ground distance sensor between a near center in the longitudinal direction of a air frame and a near center of gravity.
CONSTITUTION: A ground distance sensor 7 is arranged at the part between the center C in the longitudinal direction of a air frame 2 and the center G of gravity. The ground distance sensor 7 irradiates ultrared ray in the direction at right angle with the axis line in the longitudinal direction of the air frame 2, and measures the height from the arrival time of the reflected light supplied from a ground surface. The measurement error due to the change of the behavior of the ground distance sensor 7 itself can be reduced, reducing the reduction of height at the front and rear edge parts of the air frame due to the forward and rearward tilt of the air frame, and the precision in the height control can be improved by that portion. Further, since the ground distance sensor 7 is arranged in the sectional sharp variation part of the air frame 2, the bad influence for the height measurement such as the contamination of the lens of the sensor 7 by agricultural chemicals can be avoided, because the sensor 7 is set in the shadow of the air frame 2 for agricultural chemical spraying device 6, or the agricultural chemicals are supplied downward by the downward wind due to a main rotor 4, and the precision in the height control can be improved.