1. An onboard radar apparatus comprising: a receiving and transmitting device for transmitting electromagnetic waves to objects and receiving the electromagnetic waves reflected by the objects repeatedly, and a signal processing device for calculating relative ranges and relative velocities of the objects based on the transmitting electromagnetic waves and the receiving electromagnetic waves, wherein false images are judged by comparing relative velocities, calculated from differences between the relative ranges of the objects in previous measurement and the relative range s of the objects in current measurement, with said relative velocities of the objects, calculated based on the transmitting electromagnetic waves and the receiving electromagnetic waves at each measurement, and succeedingly the false images are deleted from outputs of the apparatus.