1. An image forming apparatus comprising: a reading device adapted to read a document; an image forming unit adapted to form a toner image based on an image of the document read by the reading device, and control a contrast potential to be used for forming a toner image by the image forming unit according to input image data, the contrast potential being a difference voltage between (1) a surface potential of a photosensitive member charged by the image forming unit and exposed by a predetermined output of the image forming unit and (2) a developing bias to be applied for developing the photosensitive member with a toner by the image forming unit; a control unit adapted to print out a test sheet, a first measurement toner image being formed on the test sheet by the image forming unit; a first density detection unit adapted to, when the reading device reads the first measurement toner image on the test sheet, detect a first density of the first measurement toner image based on a reading result of the test sheet read by the reading device; a contrast potential determining unit adapted to determine a first contrast potential so that the first density of the first measurement toner image becomes a predetermined density, based on a detecting result of the first density detection unit; a second density detection unit adapted to detect a second density of a second measurement toner image formed by the image forming unit using a second contrast potential; a characteristic determining unit adapted to determine a characteristic representing a relationship of (a) the second contrast potential and (b) the second density of the second measurement toner image detected by the second density detection unit; and a correction unit adapted to correct the first contrast potential determined by the contrast potential determining unit, based on (i) the second density of the second measurement toner image formed by the image forming unit using the first contrast potential, detected by the second density detection unit and (ii) the characteristic determined by the characteristic determining unit.