Another detection device reduces the time needed for travel motion information about another vehicle obtained by an accurate detector device, and to continues supplement for the other vehicle in the event that detection is lost. All onboard controller is connected to a communication device as a first detector device for outputting the travel motion information about another vehicle, and a sensor as a second detector device. When the difference between first travel motion information from the first detection device and second travel motion information from the second detection device is equal to or less than a predetermined threshold value, and the period of continuous detection of the second travel motion information from the second detection device is equal to or greater than a predetermined threshold value, the controller determines the first travel motion information and the second travel motion information to be travel motion information for the same vehicle.