Developing device, process cartridge, image forming apparatus and developer discharge device having a discharge port

A developing device, process cartridge and image forming apparatus of a premix developing system, with which developer splashed by the transport member is not discharged from the device, fluctuations of the amount of developer discharged to the outside are not produced, and the output image quality is stable. This developing device is provided with a pocket part, which is installed in a depressed manner facing the outside in relation to the transport route formed by the transport member, and into and out of which part of the developer transported in the transport route flows. This pocket part is provided with a discharge port for discharging developer to outside the device when the surface of the developer flowing into the pocket part exceeds a specified height.