PURPOSE: To make a rear truck self-run stably and correctly in an excavator provided with the rear truck.

CONSTITUTION: A drive control device 10 controls the self-run speed of a rear truck by using signals obtained from detectors 3, 9 to be equal to the excavating speed on an excavator main body 1 so that the distance between the excavator main body 1 and the rear truck is keps to a fixed value. When a detection error and a control error are accumulated to generate a deviation, the deviation is erased by correcting the speed, so that the distance between the excavator main body 1 and the rear truck 4 can be kept from being largely changed.