Abstract: The toner density control device of the invention is adapted to an image forming apparatus in which a plurality of processes for image formation are executed, and includes a toner sensor for detecting toner density in a developing section of the image forming apparatus, toner supplying means for supplying toner to the developing section according to output of the toner sensor, and cancelling means for temporarily cancelling function of the toner supplying means or output of the toner sensor in response to switching of operations in the process for image formation. Even if a meachanical vibration is generated during switching of operations in the process for image formation, the function of the toner supplying means or the output of the toner sensor is temporarily cancelled, so that an erroneous detection of toner density or inappropiate toner density control based on the aforesaid errorneous detection can be avoided.

Abstract: A device for detecting the life of a process unit which is detachably mounted in an image forming apparatus includes a sensor for detecting the density of the toner in the developing device and a signal detector receiving an output from the sensor and operable to determine whether the output exceeds or is less than a predetermined value during a predetermined time period which starts after a developing roller of the process unit starts rotating. A control is connected to the signal detector and is operable to effect different operating conditions depending on whether the output exceeds or is below the predetermined value, one of the operating conditions being an indication that the life of the process unit has expired and another of the operating conditions being an indication that the life of the process unit is unexpired.