Each of two or more document management server stores a document and index data corresponding to the document. Of the index data stored in the two or more document management servers, index data corresponding to a document that satisfies a first retrieval condition is collected for storage. When a user requests retrieval based on a second retrieval condition, it is determined whether the first retrieval condition and the second retrieval condition are the same, and when the first and second conditions are the same, the document retrieval according to the user's instruction is performed by referring to the collected index data.