1. A similarity determination apparatus, comprising: a processor that executes a process comprising: extracting from a document a plurality of keyword arrangements that respectively include pairs of keywords; calculating features that respectively correspond to the plurality of keyword arrangements; counting a number of appearances of each of the keywords included in the document; grouping the keywords into either of a first group or a second group, based on the number of appearances, and grouping into a third group a pair of keywords both of which belong to the second group; calculating for each of pairs of keywords belonging to the third group a total of a number of appearances of one keyword in a pair of keywords and a number of appearances of another keyword in the pair of keywords; deleting from features that are included in a certain range within the document a feature corresponding to a pair of keywords belonging to the third group, the total of the pair of keywords being low compared with other totals for pairs of keywords belonging to the third group, while keeping a number of features within the certain range equal to or greater than a predetermined threshold; and determining a similarity between the document and each of other documents by comparing features of the document, the features excluding the deleted feature from the calculated features, and corresponding features of each of the other documents.