An image forming apparatus has a developing device containing therein a developer including a toner and a carrier, the developing device having a first chamber for developing an electrostatic image formed on an image bearing member, and a second chamber constituting a developer circulating route between it and the first chamber, a carrying member rotatably provided in the second chamber for carrying the developer, a rotary member holding the developing device and rotatable in a route including a developing position, a supplying device for supplying the developer to the developing device when at the developing position, an opening provided in the second chamber for outwardly discharging therethrough any excess developer resulting from the supply of the developer, and a wall portion provided in the second chamber for suppressing the developer flowing out through the opening with the rotation of the rotary member.