An image forming apparatus capable of correcting image density promptly. A developing device supplies toner to an electrostatic latent image on a photosensitive drum by development contrast potential to form a toner image. An image density sensor detects the density of a patch image on the photosensitive drum. A toner density sensor detects the toner density of developer in the developing device. The amount of toner supplied for replenishment is adjusted when the detected toner density is above a predetermined upper limit value or below a predetermined lower limit value. Further, when the detected patch image density is above a predetermined upper limit value, toner replenishment control is switched to development contrast control to increase image density. When the detected patch image density is below a predetermined lower limit value, the toner replenishment control is switched to the development contrast control to reduce image density.