Patent ID: 8923715

Claim:
An image forming apparatus comprising: a developing device; a toner supply device that supplies toner to the developing device; a toner supply detecting sensor that detects the toner supply to the developing device; a toner concentration controller that directs the toner supply device to perform toner supply to the developing device when a toner concentration of a developer in the developing device becomes lower than a predetermined level; a toner empty determiner for determining that no toner is left in the toner supply device when the toner supply detecting sensor does not detect the effect of toner supply after the direction of the toner supply from the toner concentration controller; and, a sheet conveyance detecting sensor for detecting the paper that passes by a predetermined position in a paper conveyance path through which paper is conveyed, wherein the developing device comprises: a developer container for storing a developer including the toner and a magnetic carrier; a developer conveying structure disposed inside the developer container for agitating and circulatively conveying the developer; a developing roller for supplying the toner contained in the developer to a photoreceptor drum; and, a toner supply port that leads the supplied toner into the developer container, and, the toner concentration controller controls the operation of the toner supply device during a toner supply operation-permitted period, and when the sheet conveyance detecting sensor detects that a last sheet of an image printing being executed has passed by the predetermined position in the sheet conveyance path, the toner concentration controller controls the toner supply device so as to stop the toner supply to the developing device, and, the toner empty determiner, when an average value of the output from the toner supply detecting sensor over one cycle of a helical blade of the developer conveying structure is sampled for a fixed period of time immediately after the start of the supplying operation of the toner supply device, calculates a difference between a maximum value and a minimum value, during the toner supply operation-permitted period, and the toner empty determiner interrupts a printing job based on the difference between the maximum value and the minimum value, and the toner supply operation-permitted period is specified as the duration from a time when a command of toner supply to the developing device is given from the toner concentration controller to the toner supply device, to a time when it is determined by the sheet conveyance detecting sensor that the last sheet has passed thereby.