Developer device that gradually replaces degraded developer with fresh developer

摘要

A developer device is provided with a developer vessel for storing therein a developer material composed of toner particles and carrier granules. In the developer device, a target ratio of toner particles for a detected quantity of the developer material is computed. In the device, a toner concentration detector sensor is also provided for detecting the ratio of toner particles in the developer vessel. In order to set the detected toner ratio of toner particles equal to the target ratio, quantity of toner particles to be additionally supplied to the developer vessel is controlled. As a result, the ratio of toner particles to the developer material stored in the developer vessel can be maintained in a predetermined ratio, thereby ensuring a desirable image quality.