1. An orthographical variant detection apparatus comprising: a term extraction unit that extracts terms from document data; a similarity computation unit that computes an edit distance of an arbitrary pair of the extracted terms based on the number of times of operations for editing one of the pair to match the other of the pair, the edit distance of a character type replacing operation being set to be shorter than respective edit distances of other operations, and computes a similarity of the pair based on the edit distance; an orthographical variant candidate determination unit that determines, based on the similarity, whether or not the pair of terms are orthographical variant candidates; and a group classification unit that groups the orthographical variant candidates based on a character string commonly included in the pair of terms as the orthographical variant candidates.