1. A method of retrieving image information from a database, comprising the steps of: entering a retrieving keyword from a retrieval display unit for processing data input and output; reading an index, representing position information corresponding to said retrieving keyword by using said retrieving keyword, from a data file unit storing image data; retrieving candidate image data first by using said index; retrieving detailed image data next by using said index; wherein said step of retrieving said candidate image data and said step of retrieving detailed image data constitute a two-stage retrieving step, and wherein said two-stage retrieving step retrieves image data corresponding to said index from said data file unit; and displaying said retrieved image data on said retrieval display unit; wherein said step of retrieving said detailed image data includes a step of representing by a polynomial information a quadrangle area of said image data stored in said data file unit, said quadrangle area being defined by four corner coordinates thereof, and a step of judging through a polynomial judgment whether said retrieving keyword is included within an area defined by said polynomial.