Image forming apparatus

An image forming apparatus comprising:

• an image forming section including a plurality of photoreceptor members and two-component system developing devices which correspond respectively to each of the plurality of photoreceptor members, each of the developing devices internally houses a developer including a toner and a carrier, and supplies the toner to a surface of the respective photoreceptor member to form a toner image;
• an intermediate transfer belt onto which the toner images formed on the photoreceptor members are transferred;
• a patch density detecting section which detects a density of a toner patch formed on a surface of the intermediate transfer belt;
• a drive motor which generates a driving force;
• a driving force transmission mechanism which transmits driving force of the drive motor to each developing device and assumes a coupled state in which the driving force transmission mechanism is coupled to the developing device and transmits driving force of the drive motor to the developing device and a separated state in which the driving force transmission mechanism is separated from the developing device and does not transmit the driving force of the drive motor to the developing device;
• a toner agitating mechanism which is driven by the driving force of the drive motor when the driving force transmission mechanism is in the coupled state and which agitates the toner inside the developing device;
• a permeability sensor which detects permeability inside the developing device and outputs a signal according to a ratio of the toner to the carrier;
• and
• a coupling judging section which judges that the coupled state exists when a variation of a signal value outputted by the permeability sensor equals or exceeds a predetermined value and judges that the separated state exists when the variation of the signal value outputted by the permeability sensor is lower than the predetermined value, wherein the coupling judging section judges that the coupled state exists when the variation of the signal value outputted by the permeability sensor equals or exceeds the predetermined value and, at the same time, the density of the toner patch detected by the patch density detecting section equals or exceeds a predetermined density.