1. A similarity detecting apparatus which detects similar data from search object data based on an optional similarity criterion, comprising: means for retaining a table related to random number data generated based on a parameter for setting the similarity criterion; means for registering and managing the search object data on the table; and means for carrying out a similarity detection by using the table based on the criterion of similarity in response to a nearest neighbor search request from an optional reference point, and outputting data detected to be a nearest neighborhood.