A developing device for developing an electrostatic latent image formed on an image bearer with a dual-component developer, includes: a developing hopper for storing the developer; an agitation and conveying member for conveying the developer while agitating; a developer support which supports the developer that has been mixed and agitated inside the developing hopper and supplies the developer to a developing area located opposing the image bearer; a layer thickness-regulating member for regulating the layer thickness of the developer being conveyed by the developer support; a flow-guide plate member for flowing down the surplus developer that was rejected and conveyed away from the layer thickness-regulating member; a multiple number of diffusing elements arranged upright on the flow-guided plate member for flowing down the surplus developer in a predetermined direction whilst diffusing the developer with respect to the longitudinal direction of the developer support; and a downflow direction turning mechanism for periodically turning the downflow direction of the surplus developer regulated by the diffusing elements.