Art is provided that is capable of easily performing update processing in a short time even when updating numerous data of a storage medium of a navigation device. A directory structure corresponding to a partition structure of a storage medium is created. Image files IF are generated for updated data content within partitions HDn (n=0, 1, 2, 4, 6). The generated image files IF of the partitions HDn are stored in directories DIR that correspond to the partitions HDn in the created directory structure. Data content of the image files IF read from the directories DIR that correspond to the partitions HDn is written to the partitions HDn of the storage medium.