PURPOSE: To improve power efficiency against different load conditions to forward propulsion and steering by providing a pair of independently controllable power sources or motors as a main motor power in a drive system of track-laying vehicles.

CONSTITUTION: Separately independently controllable motors M1, M2 are output through shafts 10, 22, pinion gears 14, 24, and planetary gears 20, 30 to right and left output shafts 32, 34 to drive respective endless tracks by a sprocket. When a first range brake B1 is released and a second range clutch CII is put off, the motors are in an idling condition. When the brake B1 is engaged, the motors are developed to a first constant speed range, and when the brake B1 is released and the clutch CII is put, the motors are developed to a second speed range. When the motors M1, M2 reversely rotate, the motors are developed to reverse propulsion. Steering operation increases output speed of one motor, and decreases the same output speed of the other motor. Thereby the drive system can effectively use driving power or the motors against load conditions of the forward propulsion and the steering.