A tandem-type image forming apparatus forms a color image by transferring images formed by a plurality of image forming units set along a transfer belt onto a recording sheet transported by the transfer belt. The tandem-type image forming apparatus includes a plurality of detecting units, each detecting a toner image formed by a corresponding image forming unit to obtain the amount of toner in the toner image, a standard pattern formation controlling unit for controlling one of the image forming units to form a standard pattern to be detected by the detecting units, a sensitivity obtaining unit for obtaining a value as an evaluation value that is outputted by each of the detecting units on detecting the standard pattern, and an detection output adjusting unit for adjusting a value outputted by the detecting unit in accordance with the evaluation value. An image forming condition is set for each of the image forming units in accordance with the adjusted value, and image formation is performed under the image forming conditions.