1. An altitude reliability determining system, comprising: a controller that: acquires map information including road information and background information, the road information including road width data and the background information including background type data; acquires altitude information indicating an altitude of each unit region, the unit region being set by dividing a map indicated in the map information into a plurality of unit regions having a predetermined size; calculates a predicted altitude of each of a plurality of reference points based on the road information and the altitude information, the plurality of reference points being set along a road; accesses the road information and the background information at a position of each of the reference points; and determines a predicted altitude reliability of each of the reference points based on at least one of the background type data and the road width data at the position of the each of the reference points.