1. An image formation apparatus to form an electrostatic latent image on an image supporting object, the electrostatic latent image being developed with a toner, the image formation apparatus comprising: a developing unit; a toner supply unit to supply the toner to the developing unit; a patch image density detecting unit to detect a density of a patch image formed on the image supporting object; a toner density detecting unit to detect a density of the toner held in the developing unit; and a control unit configured to determine an amount of difference between the density of the patch image and a predetermined target density value, and to select one of a plurality of generating intervals for formation of the patch image based upon a comparison between the amount of difference and a corresponding one of a plurality of ranges of difference between the density of the patch image and the predetermined target density value, so that an image density value converges to the predetermined target density value.