1. A semantic network machine comprising: first associative memory means for storing a plurality of knowledge data arranged to form a semantic network, thereby constituting a knowledge base, each of the knowledge data having a set of an object, an attribute, and a value, and the attribute including an "is-a" attribute; second associative memory means, arranged in parallel with said first associative memory means, for storing a plurality of specific knowledge data including the "is-a" attributes of the plurality of knowledge data; and retrieval control means, connected to said first and second associative memory means, for parallel-accessing said second associative memory means while accessing said first associative memory means so as to retrieve knowledge data necessary for answering an input question in an inferential retrieval mode, for, when the accessing in said first associative memory means fails, feeding back an access result, which represents another object concept associated with an object concept included in the input question by the "is-a" attribute, from said second associative memory means to said first associative memory means, and for successively accessing said first associative memory means using the access result as a part of a new retrieval condition.