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, 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, a display; a display controller configured to cause the display to display the character converted by the converter and a result of mark detection performed by the determination part with respect to the converted character; an instruction acceptance part configured to accept a character setting instruction representing whether the character displayed on the display is set to be the first reference character; and a setting part configured to perform one of: setting the character displayed on the display as the first reference character; and excluding the character displayed on the display from the first reference character, according to the character setting instruction accepted by the instruction acceptance part, wherein the determination part stores in advance a 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.