An object of the present invention is to provide an antilock brake system control device that can realize an accurate constant speed running control by cooperating an ABS control and a constant speed running. The control device includes: an ABS control hydraulic circuit that controls an ABS of each wheel; constant speed running controllers 213, 271 that supply hydraulic pressure to a rear brake 12 when a vehicle runs down a slope; wheel rotation speed detectors 13FL, 13FR, 13RL, 13RR; a monitor 67 that monitors a set speed under constant speed running control; a monitor 61 that monitors actual speed of the vehicle; a judging unit 68 that judges whether running speed is being kept constant or not based on the set speed and the actual speed; and a regulator 69 that regulates decompression by the ABS control when it is judged that the running speed is not being kept constant.