To provide an image forming device capable of correcting gradations so that the gradations are appropriate for outputting a variety of images regardless of alterations of image forming conditions.

The image forming device has a test image data generating means which generates test image data of specific pixel density, a density detecting means which detects the density of a test image developed in accordance with the test image data, and a correction characteristic calculation means that calculates gradation correction characteristic in order to correct exposure time of a laser light source based upon the detected density information. The gradation correction characteristics are calculated by forming a first test image group and a second test image group. The first test image group is composed of a plurality of test images exposed under different exposure conditions. The second test image group is composed of a plurality of test images exposed under different exposure conditions while using an image formation mode switched so as to be different from the one used for the first test image group.