When a number of document data registered in one of full-text index-for-registration/deletion storage parts reaches a predetermined number, or when a capacity of the full-text index-for-registration/deletion storage part reaches a predetermined capacity, a merge processing is performed for merging data from the full-text index-for-registration/deletion storage part to a full-text index-for-search storage part. While this merge processing is performed, a registration/deletion processing is performed by using another full-text index-for-registration/deletion storage part.