A navigation device realizes such processing as map display and route guidance based on map data stored in a memory card. The data in the memory card tends to be volatilized with an increase in the frequency of reading of data. Therefore, the data that are highly frequently read out are held in a RAM so as to be read from the memory card at a decreased frequency. Further, the passage of time is calculated from the date and hour the data are recorded in the memory card, and the whole data in the memory card are refreshed every time when the passage of time exceeds a threshold value T1.