1. A graph structured data processing method, for extracting, from a graph database constituting a set of graph structured data, a frequent graph that has a support level equal to or greater than a minimum support level, comprising: a first step of generating, from a set F k of adjacency matrixes of a frequent graph having a size k (k is a natural number), a set C k&plus;1 of adjacency matrixes c k&plus;1 of candidate frequent graphs having a size k&plus;1; a second step of deleting, from said set C k&plus;1, said adjacency matrix c k&plus;1 of a candidate frequent graph that includes a less frequent graph as an induced subgraph having said size k; a third step of selecting only one adjacency matrix c′ k&plus;1 from a sub-set of adjacency matrixes c k&plus;1 that represent the same graph; and a fourth step of, when said adjacency matrix c′ k&plus;1 is a frequent graph as the result of scanning of said graph database, adding, to a set F k&plus;1 of adjacency matrixes of a frequent graph having said size k&plus;1, said adjacency matrix c′ k&plus;1 and an adjacency matrix c k&plus;1 that represents the same structure as a graph expressed by said adjacency matrix c′ k&plus;1, wherein, at said first step of generating said adjacency matrix c k&plus;1, adjacency matrixes X k and Y k of a frequent graph that is a generation base are coupled together by the establishment of a first condition, wherein between said matrixes X k and Y k elements other than the k-th row and the k-th column are equal, a second condition, wherein between graphs G(X k ) and G(Y k ), which are represented by said adjacency matrixes X k and Y k, vertex levels other than the k-th vertex are equal and the order of the level of said k-th vertex of said graph G(X k ) is equal to or lower than the order of the level of the k-th vertex of said graph G(Y k ), a third condition, wherein between said graphs G(X k ) and G(Y k ) the vertex level at the k-th vertex is equal and the code of said adjacency matrix X k is equal to or smaller than the code of said adjacency matrix Y k, and a fourth condition, wherein said adjacency matrix X k is a canonical form.