1. A searching method comprising: an extraction step of extracting extracted documents, each of which contains a desired search string, from a plurality of documents which is included in each of a plurality of document groups in such a manner that, regarding at least one of the plurality of document groups, a plurality of documents is extracted from one document group; a preference determination step of determining output preferences, each of which is determined for each of the extracted documents including the plurality of documents extracted from the one document group, in accordance with a matching level between the search string and character strings in each of the extracted documents; an output preference specification step of specifying one of the output preferences determined at the preference determination step; a document group specification step of specifying one document group among the plurality of document groups; a document determination step of determining whether a document for which the output preference specified at the output preference specification step was determined is contained in the one document group specified at the document group specification step; an output step of outputting the document that was determined to be contained at the document determination step; a first repetition step of repeating the document group specification step, the document determination step and the output step while sequentially changing the one document group specified at the document group specification step, as long as another document for which the output preference specified at the output preference specification step was determined remains in any of the plurality of document groups, in a state of not being output; and a second repetition step of repeating the output preference specification step, the document group specification step, the document determination step, the output step and the first repetition step while changing the output preference specified at the output preference specification step one by one in an order from a highest preference to a lowest preference.