VEHICLE CONTROL DEVICE, VEHICLE CONTROL METHOD, AND VEHICLE CONTROL SYSTEM

When a preceding vehicle in front of a host vehicle reduces speed, other vehicles between the preceding vehicle and a system-mounted vehicle 100 successively reduces speed. An ECU 20 and an ACC 30 control the traveling of the system-mounted vehicle 100 on the basis of prediction information of speed reduction propagation when the system-mounted vehicle 100 reduces speed such that the system-mounted vehicle 100 and an ordinary vehicle 200 immediately in front of the system-mounted vehicle 100 have a predetermined relative positional relationship. Therefore, it is possible to suppress the speed reduction propagation between the preceding vehicle and the system-mounted vehicle 100 in advance, making it possible to more effectively suppress congestion.