1. A method for switching a reference map data in a navigation device having a map data memory for storing a plurality of map data, the navigation device executing a navigation function with using a first map data among the plurality of map data stored in the map data memory as the reference map data the method comprising: reading out a second map data from a memory medium; determining whether the map data memory already stores a same map data as the second map data; and switching the reference map data from the first map data to the same map data as the second map data when the map data memory already stores the same map data as the second map data in the memory medium.