1. A mark detection apparatus, further 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; 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 the 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 converter comprises a process with an optical character reader, wherein the determination part is configured to determine that the portion of the image read by the reading unit is a mark in a case in which the converted character converted by the converter coincides with the first reference character, and that the portion of the image read by the reading unit is not a mark in a case in which the converted character converted by the converter coincides a preset second reference character that is different from the first reference character, the display controller is configured to cause the display to display the converted character for which the determination part determines that a mark is indeterminable, the instruction acceptance part is configured to accept the character setting instruction instructing to which of the first reference character and the second reference character the character displayed on the display is assigned, and the setting part is configured to set the character displayed on the display as the first reference character or the second reference character, according to the character setting instruction accepted by the instruction acceptance part.