1. A document retrieving system for retrieving a document from documents registered in two or more document management servers which are connected via a network, said document retrieving system comprising: a storing unit, provided with each of the document management servers, configured to store a document and index data corresponding to the document; a collection unit configured to collect the index data corresponding to a document satisfying a first retrieval condition from the index data stored in said storing unit of each of the document management servers; a determination unit configured to determine whether a second retrieval condition is the same as the first retrieval condition, the second retrieval condition being designated by a user; and a retrieving unit configured to retrieve the document by referring to the index data collected by said collection unit, in a case where said determination unit determines that the second retrieval condition is the same as the first retrieval condition.