An image forming apparatus includes a developing device assembly, an inertia acceptor and a setting mechanism. The developing device assembly has a plurality of developing devices to develop a latent image formed on a photosensitive member. The assembly can rotate to switch from one of the developing devices to another. The energy of rotational inertia is shifted from the developing device assembly to the inertia acceptor when the inertia acceptor stops the rotation of the developing device assembly. The setting mechanism controls the movement of the inertia acceptor driven by the energy of rotational inertia shifted from the developing device assembly.