To achieve a new method for defining related words.

A navigation server which communicates with a navigation system for guiding a movable body to a destination by displaying a current position of the movable body on a map on a display screen of a display device is provided with a POI information management DB 23, acquires a retrieval character string used for retrieval of POI information from the navigation system, performs retrieval for searching POI information including the acquired retrieval character string from among the POI information stored in the POI information management DB 23, registers the retrieved POI information and the retrieval character string in association with each other, in a prescribed memory device, determines a relevance level between the registered POI information and the retrieval character string based on registration frequency, specifies an associative character string which is a character string in which the relevance level to the acquired retrieval character string is associated with the POI information satisfying a prescribed condition are registered in association with each other, retrieves the POI information by using the specified associative character string, and sends the retrieval result to the navigation system.