1. An object detecting device comprising: a detection unit that detects the relative velocity of an object, which exists in the lateral direction of a vehicle, with respect to the vehicle; a velocity calculating unit that calculates the absolute velocity of the object in the horizontal direction of the vehicle on the basis of a detection signal of the detection unit; and a determination unit that determines whether or not the object is an approaching moving object on the basis of the absolute velocity of the object in the horizontal direction of the vehicle calculated by the velocity calculating unit.