1. A frequent changing pattern extraction device comprising: a conversion unit configured to convert 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.