1. A coupled node tree splitting method comprising: preparing step that prepares a coupled node tree used which is in a bit string search and comprises a root node and a node pair, the node pair being a branch node and a leaf node, or a pair of branch nodes, or a pair of leaf nodes located in adjacent storage areas, wherein the root node is a node that expresses a starting point of the tree and, which is a leaf node when there is one node in the tree and a branch node when there are two or more nodes in the tree, the branch node includes a discrimination bit position of a search key with which a bit string search is performed and position information indicating a position of a primary node, which is one node of a node pair of a link target, and the leaf node includes an index key that is a bit string that is the target of a search, and wherein repeating linkage, at the branch node, to a primary node or a node at a position in a memory area adjacent thereto of a node pair of the link target from an arbitrary node of the tree as a searching start node in accordance with a bit value of a search key of a discrimination bit position included in the branch node, until the leaf node is reached, an index key stored in the leaf node is made a search result key, which is a search result using the search key of an arbitrary subtree having the searching start node as its root node; a step of obtaining a split key that establishes an index key that splits a processing source coupled node tree that is to be split; a processing source minimum value or maximum value obtaining step that determines a minimum value or a maximum value of the index keys of the processing source coupled node tree; a comparing step of comparing the split key with either the minimum value or the maximum value and, if the minimum value is larger than the split key or if the maximum value is smaller than the split key, ending processing; a generating step of, if the result of the comparison is that the minimum value is not larger than the split key or the maximum value is not smaller than the split key, inserting the minimum value or the maximum value index key, so as to generate a new processing target coupled node tree; a deleting step of deleting the minimum value or maximum value index key from the processing source coupled node tree; and wherein by making the processing source coupled node tree from which has been deleted the minimum value or maximum value index key into a new processing source coupled node tree, the processing source minimum value or maximum value obtaining step, the comparing step, the generating step, and the deleting step are repeated until the minimum value or maximum value obtained by the processing source minimum value or maximum value obtaining step becomes larger than or smaller than the split key respectively.