1. An associative memory which stores mask information for every single word of storage data to indicate a valid state or an invalid state representative of whether or not every single bit or every plural bits of said storage data should be excluded from a search object; said associative memory comprising: i) a first circuit means as a primary searching means for producing a primary distinguishing signal to distinguish a selected word, when all bits of an external search data composed of partial bits, n in number (where n is an integer variable 2 or more), are in a primary searching operation, said primary searching operation taking said mask information into account in order to exclude single or plural bits of said storage data from a search object when said corresponding mask information is in a valid state of mask information for each single word, ii) a second circuit means for producing an intermediate data which supply an intermediate data produced by a logical operation of storage data corresponding to words in a selected state, a logical operation of mask information corresponding to said words, and a logical operation of a single or plural pairs of external search data, iii) a third circuit means as a single or plural shared secondary searching means for producing a secondary distinguishing signal which distinguish a word selected from said search operation, when said intermediate data divided into n, n in number, are supplied similar to a composition of partial bit fields of said external search data, controlling whether or not a secondary search operation is to be carried out against only a bit field of said storage data or mask information corresponding to said partial bit fields in words, depending on a state of secondary search controlling signal, iv) a fourth circuit means for updating and storing a word invalid information at each primary and secondary search operation, whether a word corresponding to each word is selected or not, through primary and secondary search operation, and providing said selected state or non-selected state to said intermediate data producing means, and v) a fifth circuit means for controlling, to produce said secondary search controlling signal to implement said secondary search, using said partial bit field, n in number, of said intermediate data as a search data in order, after said primary search operation using all bit fields of said external search data as a search data.