1. An image forming apparatus, comprising: a patch pattern forming part to form an image of a patch pattern on an image carrier; a developing part to develop the image of the patch pattern; an image density detecting part to measure an image density of the patch pattern; and a process control performing part to vary an image forming process condition based on detection results obtained from the image density detecting part, and to stabilize the image density, wherein, corresponding to various factor information, the process control performing part is configured to set a gradation number of the image of the patch pattern and the detection results obtained from the image density detecting part to stabilize the image density are associated with a development gamma of a halftone of the patch pattern.