second area called u201c assumed ghost area u201d hereinbelow are designated the first area is an area where a vehicle running on an adjacent lane at a relative speed with respect to a predetermined vehicle can at least once be detected the assumed ghost area is defined to be an area that is different from the first area in the detection area an object not detected in the first area and detected in the assumed ghost area for the first time is judged to be a ghost fig3 is a diagram for illustrating how an assumed ghost area is designated in accordance with one embodiment of the invention a predetermined vehicle 100 having a radar apparatus not shown is currently running on a lane 500 there are two adjacent lanes 700 adjacent to the currently running lane 500 the radar apparatus has a detection area 120 where an object can be detected the detection area 130 has a side boundary 110 and a front boundary 130 assuming that the range of speeds detected by the radar apparatus on the predetermined vehicle 100 is given as vmin to vmax and that a period of a detection cycle control cycle for detecting an object by the radar apparatus is designated as u03b4t the distance d the predetermined vehicle 100 runs during one period is calculated by d vmax u00d7 u03b4t the detection area 130 includes a first area 400 and an assumed ghost area 40 the first area 400 is defined to be an area enclosed by the side boundary 110 of the detection area 120 the distance d on a boundary.