1. A medium computer-readably storing a document retrieval program for causing a computer to execute a process, the process comprising: acquiring first key information which is a specific type of information; retrieving first documents including the first key information from a plurality of documents in a database, each of the plurality of documents including at least one of output classification codes, the output classification codes classifying the plurality of documents; extracting codes of the output classification codes from the first documents as candidate classification codes; retrieving second documents including the candidate classification codes from the plurality of documents; extracting second key information which is the specific type of information, from the second documents; and evaluating the candidate classification codes on a basis of comparison of the first key information and the second key information; wherein the extracting of the second key information associates a first candidate classification code of the candidate classification codes with a first piece of the second key information extracted from one of the second documents including the first candidate classification code, and repeats the associating for all of the candidate classification codes, and the evaluating determines a second key information score of a second piece of the second key information on a basis of a degree of agreement of the second piece of the second key information relative to the first key information, repeats the determining of the second key information score for all pieces of the second key information so as to determine second key information scores of all pieces of the second key information, determines a candidate classification code score of a second candidate classification code of the candidate classification codes on the basis of a second key information score of a piece of the second key information corresponding to the second candidate classification code, repeats the determining of the candidate classification code score for all of the candidate classification codes so as to determine candidate classification code scores of the candidate classification codes, and uses the candidate classification code scores as an outcome of the evaluating.