To realize a high speed merge sort method by applying a coupled node tree, which method extracts a smallest or largest key from a plurality of sorted key storage areas in each of which is stored keys including bit strings that are sorted, and generates a coupled node tree for merge while adding a processing source identifier that identifies the sorted storage area wherefrom the key has been extracted, and repeats the actions of writing out into the merged key storage area a key being obtained by a minimum or maximum value search on the coupled node tree and deleting the key, and inserting into the coupled node tree a key by extracting the key from one of the plurality of sorted key storage areas.