1. A reference data recognition and learning method of realizing reference data recognition and learning with respect to an associative memory, which compares inputsearch data with several reference data to select and output the most similar reference data, comprising: a rank giving step of giving a rank corresponding to a priority to several reference data stored in the associative memory; a selecting step of comparing the search data with said several reference data every time when the search data is input to detect a similarity, and selecting a reference data having the highest similarity; an identity determining step of determining whether or not the search data and the reference data selected by the selecting step are identical to each other from their similarity; and a rank control step of improving a rank of the corresponding reference data by a predetermined number while putting back a rank of other reference data when it is determined that both data are identical in the identity determining step, and registering the search data to a predetermined rank of the reference data while putting back a rank of other reference data equal or lower than the rank when it is determined that both data are not identical in the identity determining step.