To group ID tags distributed in a larger area than the coverage of a tag reader, a parameter which represents a measure of the strength of relationship among a plurality of ID tags is stored in a parameter storage database. Values of the parameter are increased and decreased according to the number of pieces of ID information detected simultaneously in the ID tags by a mobile tag reader. A plurality of ID tags are judged as belonging to the same group according to values of the parameter stored in the parameter storage database, thereby making it possible to group ID tags distributed in a larger area than the coverage of a tag reader.