The static latent image formed on the photoreceptor which has been charged, by exposing it to light, is developed through four colors of developing units, i.e., yellow, magenta, cyan and black, forming toner images on the photoreceptor. These toner images are transferred to the transfer drum. Before forming the toner images, a device is provided to correct the measurement value of toner density obtained using the light emitter and photosensor into an exact value. Based on the reflected light from a white-coated area formed on the transfer drum, two levels of the intensities of emitted light are determined. The intensity of received light is compared to the standard value so that the output value from the photosensor corresponding to the toner density is made to approximate the standard value.