1. An image forming apparatus comprising: a first image forming unit that forms toner images based on image data on a plurality of first image carriers; a second image carrier, which moves at a predetermined speed and on which the toner images formed on the first image carriers by the image forming unit are transferred; a second image forming unit that transfers the toner images transferred on the second image carrier onto a transfer medium that is moved at the predetermined speed; a test pattern generating unit that generates a test pattern group with a predetermine length in a moving direction of the second image carrier; and an adjusting unit that determines a method to adjust an image formation condition for the first image forming unit using the test pattern group, based on a relationship between the predetermined length and a length of an image area, in which a print image based on the image data is formed, in a sub-scanning direction corresponding to the moving direction of the second image carrier.