8. A document retrieval system as claimed in claim 7, wherein said relevance value is a value within a range which is equal to or greater than 0 and is equal to or less than 1, and said evaluation value is an arbitrary value within a range which is equal to or greater than 0 and is equal to or less than 1.
8. A document retrieval system as claimed in claim 7, wherein said relevance value is a value within a range which is equal to or greater than 0 and is equal to or less than 1, and said evaluation value is an arbitrary value within a range which is equal to or greater than 0 and is equal to or less than 1. 9. A document retrieval system as claimed in claim 8, wherein said learning means has first means for modifying one or a plurality of said relationship values in said keyword connection table when said relevance value obtained by said calculation means is equal to 0, and second means for modifying one or a plurality of said relationship relation values in said keyword connection table when said relevance value obtained by said calculation means is not equal to 0.