The invention provides a developer having at least a toner, an aeration ratio (AR) of the developer measured by a powder rheometer being in a range of about 5.0 to about 10.0. It is preferable that the developer further contains an external additive of silica particles having a small diameter. The invention further provides a process cartridge having at least: a photoreceptor; and a developing device that comprises plura developer storage portions and that makes visible a latent image formed on the photoreceptor by using the developer. The invention further provides an image forming apparatus having at least the process cartridge.