An image forming apparatus includes a rotatable developing device incorporating first developer units, a second developer unit disposed outside the rotatable developing device, a photoconductor on which a toner image can be formed by contact of each of the first and second developer units, and a driving motor for rotating the rotatable developing device. A first value of angular acceleration of the rotatable developing device when development performed by a first one of the first developer units proceeds to development performed by the second developer unit and a second value thereof when the development performed by the second developer unit proceeds to development performed by a second one of the first developer units are set to be smaller than a third value thereof when development performed by the second one of the first developer units proceeds to development performed by a next one of the first developer units.