1. An image forming apparatus, comprising: a movable image bearing body; image forming means for forming a developer image on the image bearing body; transferring means for transferring the developer image formed on the image bearing body onto a moving transferring medium; and control means for controlling the image forming means to form a predetermined image composed of dot images with a predetermined density prior to formation of a normal image, wherein the control means controls the image forming means so as to: form a composite image from the normal image and the predetermined image in an area where the normal image is to be formed; and form the dot images in a dot area having the normal image and the predetermined image overlapped with each other, with a density determined on the basis of a relationship between a density of the normal image and the predetermined density in the dot area.