An image forming apparatus, including: an image bearing member; a charging device configured to charge a surface of the image bearing member; a power source configured to apply a voltage to the charging device; an exposure device configured to irradiate the surface of the image bearing member with a light beam to form an electrostatic latent image; a developing device configured to develop the latent image into a toner image; and a reading device configured to read a color registration toner image obtained by developing, by the developing device, a color registration electrostatic latent image formed on the surface of the image bearing member by the exposure device within a period from a time when the power source is started up to apply the voltage to the charging device to a time when a potential of the surface of the image bearing member reaches a potential for usual image formation.