To appropriately provide data required for updating map data to an information terminal.

As data for updating map data, instruction data is transmitted from a distribution system to a navigation device, the instruction data containing an ID code of update target data and detailed instruction data for each update target area in the update target data. The detailed instruction data contains an offset value capable of specifying the update target area and overwrite data to overwrite in the update target area. The navigation device processes the detailed instruction data items contained in the received instruction data in the order of array and updates the map data by writing the overwrite data into the update target area indicated by the detailed instruction data of the map data that the navigation device comprises. In the detailed instruction data, there is described as an offset value a relative position from a spot in the update target data indicated by an offset value of the detailed instruction data preceding in the order of array. The description suppresses the offset value and a data size of the instruction data.