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 an external memory medium; determining whether the map data memory in the navigation device already stores a same map data as the second map data; switching the reference map data from the first map data to the second map data already stored in the map data memory of the navigation device without installing the second map data from the external memory medium, when the map data memory already stores the same map data as the second map data in the external memory medium; determining whether the same map data as the second map data is used as the reference map data before the switching of the reference map data from the first map data to the same map data as the second map data; receiving the external memory medium inserted into a slot of the navigation device before the reading out of the second map data; writing the second map data in a second region of the map data memory in the navigation device different from a first region of the map data memory in the navigation device, in which the first map data is stored, when the map data memory in the navigation device does not store the same map data as the second map data; and switching the reference map data from the first map data to the second map data, wherein the switching of the reference map data from the first map data to the same map data as the second map data is performed when the same map data as the second map data is not used as the reference map data; wherein the switching of the reference map data from the first map data to the same map data as the second map data is not performed when the same map data as the second map data is used as the reference map data, wherein the navigation device executes the navigation function with using the first map data stored in the first region of the map data memory in the navigation device while the navigation device is writing the second map data in the second region of the map data memory in, the navigation device.