Disclosed is an image forming apparatus, which comprises an intermediate transfer belt (15), and a plurality of image-forming units (11) to (14) disposed along the intermediate transfer belt (15) and adapted to form toner images of different colors and sequentially transfer the toner images onto the intermediate transfer belt (15) in a superimposed manner so as to form a color toner image. The intermediate transfer belt (15) is laid across a drive roller (16), a driven roller (17) and a tension roller (18), in a tensioned condition. A rotation speed of the tension roller (18) adapted to apply a tension to the intermediate transfer belt (15) is detected to obtain a linear speed of the intermediate transfer belt (15). In a process of color-misregistration correction, a write-start timing for each toner image of the colors is adjusted based on the linear speed of the intermediate transfer belt (15).