An image forming apparatus includes: a rotatable image carrier; an image forming unit configured to form a plurality of divided patch images, which is obtained by dividing a patch image for detecting a concentration of an image in a rotation direction of the image carrier, on the image carrier at regular intervals and form an image to be formed on a paper sheet on the image carrier; a concentration detecting unit configured to detect the concentration of the plurality of divided patch images formed by the image forming unit; and a controller configured to combine the concentration of divided patch images detected by the concentration detecting unit in chronological order and detect concentration unevenness of the image in the vertical scanning direction which is a paper sheet conveying direction of the paper sheet.