By converting address information 200 input to a geocoding unit 10 into a tree structure formed hierarchically in an administrative district, generating address tree information obtained by associating latitude and longitude information with at least a node of the lowest layer in each branch of the tree structure and storing the address tree information thus generated in a high-speed accessible memory 14 to proceed a retrieval while tracing the address tree information in order from a parent node to a child node to narrow down a retrieval range, it is possible to efficiently search latitude and longitude information corresponding to given address information therefrom and to perform a retrieval at a higher speed as compared with the related art in which access is given to a conversion table stored in a disk.