A terminal node the value of a discrimination bit position of which is smaller than 0 is first stored in a search route stack for storing a search history and branch nodes followed by way of the search route are sequentially stored in order to prevent the efficiency of a search processing using a coupled node tree from being reduced even if the size of the coupled node tree is increased. On that occasion, the representative node number of the branch nodes, if the link destination is node [1], is changed into the value to which 1 is added. The hit ratio of a cache can be increased in the search processing to improve the efficiency of the processing by storing the branch nodes in the search route stack.