A pixel count section calculates a coverage rate corresponding to toner consumption. A control coefficient value setting section determines, based on the coverage rate, a stop threshold value representing reference concentration for toner replenishment and replenishment extension time. A magnetic permeability sensor measures permeability representing toner concentration in a housing portion. A toner concentration control section starts toner replenishment when an output value from the magnetic permeability sensor varies from a value less than a start threshold value to the start threshold value or above, and stops toner replenishment after lapse of replenishment extension time from the time at which an output value varied from a value greater than the stop threshold value to the stop threshold value.