1. A similarity determination apparatus, comprising: a processor that executes a process comprising: counting a number of appearances of each keyword included in a piece of document information; deleting any arrangement including a keyword having the number of appearances less than a threshold under a condition where a number of types of keyword arrangements included in a certain range of the piece of document information is equal to or greater than a certain number; extracting, as features, a plurality of keyword arrangements from the piece of document information; and determining a similarity between the different pieces of document information by comparing the features extracted from pieces of document information different from each other.