PURPOSE: To excavate the ground with a maximum earth-moving efficiency by a method in which on the basis of the characteristic curve of soil amount to be shown from the product of excavating depth and vahicular speed, maximum excavating depths at each excavating point are obtained, and control on feedback is repeated.

CONSTITUTION: The soil amount Q has a characteristic curve to be varied with the vehicular speed V of a tractor and the depth H of excavation. An excavating depth detector 10 is attached to working tool 1 of a construction machine, and a vehicular speed detector 40 is provided to the tractor. For characteristic curve H×V composed of the speed V and the depth H detected, the ultimate depth value H0 of the curve is detected, and while keeping the depth H0 by feedback control, the soil amount Q is excavated. The maximum depth value H1 of the characteristic curve H×V of the soil amount Q is obtained, and on the basis of the value H1, feedback control is made for excavating work. Afterwards, control on the excavating work is repeated likewise to maximize the characteristic curve H×V.