1. An image forming apparatus comprising: an image carrier being stretched around a plurality of rolls, the image carrier having a corrugated area where a portion of the image carrier is plastically deformed; and a detection unit that detects a density of a toner image formed on the image carrier, based on an amount of regularly reflected light from a surface of the image carrier, the detection unit configured to detect light emitted from a toner image within a detection area, wherein a detection length of the detection area in a movement direction of the image carrier is longer than a length, in the movement direction of the image carrier, of a the corrugated area where a deformation of the image carrier is caused, wherein the detection length is a length that is a sum of the length of the corrugated area of the image carrier in the movement direction of the image carrier, and a length which is twice a length of a detection area of the detection unit.