An image forming device has image forming units 3-6, a transfer belt 7, a position detection unit 19, a concentration sensor 23, and an adjustment unit. A toner image is formed on the surfaces of the image forming units 3-6, and transferred onto the transfer belt 7. The position detection unit 19 detects a predetermined position on the surface of the transfer belt 7. On the predetermined position, the concentration sensor 23 detects first information relating to the color of the surface of the transfer belt 7. The concentration sensor 23 detects second information relating to the color of the pattern image that is formed on the predetermined position. The adjustment unit adjusts toner concentration at the time of forming an image based on the first information and the second information detected by the concentration sensor 23.