1. A frequent changing pattern extraction device comprising: a processor; a conversion unit configured to convert, using said processor, a graph sequence into an operator sequence by expressing changes, from a first graph included in the graph sequence to a second graph which is temporally adjacent to the first graph, using operators indicating operations necessary to transform the first graph into the second graph, the graph sequence including a plurality of graphs that show temporal changes in the graphs, and each of the graphs including a vertex corresponding to a data piece and an edge corresponding to a link between data pieces; and an extraction unit configured to extract an operator subsequence that appears at least a predetermined number of times in the operator sequence, based on anti-monotonicity used in the Apriori algorithm, wherein there are a plurality of graph sequences, wherein said conversion unit is configured to convert each of the plurality of graph sequences into an operator sequence by expressing changes, from a first graph included in the graph sequence to a second graph which is temporally adjacent to the first graph, using operators indicating operations necessary to transform the first graph into the second graph, and wherein said extraction unit is configured to extract an operator subsequence that appears at least the predetermined number of times in the operator sequences corresponding to the plurality of graph sequences, based on the anti-monotonicity used in the Apriori algorithm.