An image forming apparatus comprises a rotatable rotating body to and from which a plurality of developer containers can be attached and detached, each of the developer containers being capable of containing developer of a different color. The Langevin type vibrator has a construction in which a front mass, a cylindrical vibrator, and a rear mass are provided in tandem, and those elements are tightened by a bolt. This cancels an influence of density variations appearing in association with rotating cycles of the photosensitive member exerted over a patch image.