1. A mark detection apparatus comprising: a reading unit configured to read an image on a document; a converter configured to compare respective portions of the image read by the reading unit with a plurality of preset reference patterns and convert a portion of the image to a character associated with a corresponding reference pattern, wherein the converter comprises a process with an optical character reader, and a determination part configured to determine that the portion of the image to have been compared on the document is a mark in a case in which the character converted by the converter is a preset first reference character, and that the portion of the image to have been compared on the document is not a mark in a case in which the character converted by the converter is not a preset first reference character, wherein the determination part stores in advance the following plurality of characters including a shape of a black square and shapes similar to the black square as first reference characters among characters converted by the converter.