In an apparatus for retrieving documents, each document and a query character string are divided into a plurality of index keys. A retrieval condition analyzing part analyses a retrieval condition including the index keys divided from the query character string and generates a retrieval condition tree synthesized by at least one operator that retrieves an intermediate retrieval result including the document information from said index table. A retrieval condition evaluating part evaluates each intermediate retrieval result obtained by the retrieval condition tree and determining a final retrieval result.