An information retrieval system is used for retrieving information from a database. The information retrieval system includes a parser for parsing a natural language input query into constituent phrases as a syntax analysis result. The system also includes a virtual table for converting phrases of the natural language query to retrieval keys that are possessed by the database. The virtual table accounts for particles that modify the phrases in the input query. A collating unit is provided in the system for preparing a database retrieval formula from the syntax analysis result by selecting a virtual table that it is used to convert the phrases to the keys possessed by the database. Lastly, the system includes a retrieval execution unit for retrieving data from the database on the basis of the database retrieval formula.