1. A full-text search device searching for document data including a search condition input from a document data storage part, which stores input document data, with the use of a first full-text index storage part for registration and a second full-text index storage part for search, the device comprising: a registration processing part which obtains tokens and appearance position information of the tokens from the document data, and stores the tokens and the appearance position information of the tokens in the first full-text index storage part for registration; a deletion processing part which determines whether tokens obtained from a document to be deleted are stored in the first full-text index storage part, and deletes the appearance position information of the tokens obtained from the document to be deleted from the first full-text index storage part when the tokens obtained from the document to be deleted are stored in the first full-text index storage part; a search processing part which obtains a first set of tokens of document data including the input search condition with the use of an index of the first full-text index storage part, obtains a second set of tokens of document data including the input search condition with the use of an index of the second full-text index storage part, and subsequently outputs, as a search result, a set obtained from a union of the first and second sets; a merging part which carries out merge processing by adding inverted lists of tokens from the first full-text index storage part to inverted lists of tokens corresponding to the index of the second full-text index storage part, and subsequently emptying the index of the first full-text index storage part.