1. An image forming apparatus comprising: an image forming portion having a multiple image forming stations, each forming a color separated image with a corresponding developer in accordance with color-separated image information; a transfer means for superimposing color separated images formed on the multiple image forming stations, one over another and transferring the superimposition; a pattern image forming means for making each image forming station form a predetermined pattern image; a pattern image density measuring means for measuring the density of the pattern image; and a timing control means which performs control in such a sequence that one image forming station to be the reference, in the image forming portion, forms multiple pattern images; one image forming station to be adjusted forms the same number of pattern images at timings which are shifted from one another by integral multiples of a predetermined unit time so that these formed pattern images are superimposed over the pattern images formed by the reference image forming station; the pattern image density measuring means measures the density of the multiple superimposed pattern images so as to judge superimposed states of the pattern images based on the measured densities; and the image forming station to be adjusted is controlled so as to perform image forming at a timing which can produce a measured density value closest to the preset value.