PURPOSE:To make a drive coupling device for four-wheel drive, miniature and lightweight, by controlling the discharged pressure of a hydraulic pump which is adapted to be driven due to the difference in rotational speed between the front wheel drive shaft and the rear wheel drive shaft, such that the drive mode is changed over between four and two wheel drive modes, and the drive power for the front and rear drive shafts are well-distributed. CONSTITUTION:A rotary shaft 11 in a differential gear unit 10 for front wheels 9 is coupled with a rotary shaft 14 in a differential gear unit 17 for rear wheels 16. This hydraulic pump 13 is driven due to the difference in speed between both rotary shafts 11, 14 and therefore, discharges oil in an amount in accordance with the speed difference. Further, the rotary shaft 14 for the rear wheel side is coupled to the output shaft 3 of an engine 1 through gears 4, 5. Further, the discharge pressure of the hydraulic pump 13 is controlled to change over the mode of drive between four and two wheel drive modes, and as well the drive power is controlled to be well-distributed to the front and rear wheels.