1. A position detection apparatus comprising: a storage unit configured to store map information including road information; a detection unit configured to detect information for calculating an absolute position of a moving object; a position calculation unit configured to calculate the absolute position of the moving object and an error variance relating to an error of the moving object from the information detected by the detection unit; a read-out unit configured to read out from the storage unit the road information of a road relating to the absolute position calculated by the position calculation unit; an existence probability calculation unit configured to calculate an existence probability of the moving object existing on the road from the absolute position and the error variance calculated by the position calculation unit, and the road information read out by the read-out unit; a selection unit configured to select a position whose existence probability is maximum out of the existence probability calculated by the existence probability calculation unit; and a map-matching processing unit configured to make the position selected by the selection unit a position of the moving object on the road.