To provide a toner replenishing amount control method and an image forming device capable of obtaining desired image density by realizing more appropriate toner replenishment by a toner concentration detection means.

An output value Vt by a permeability sensor is corrected according to a condition such as an original area rate, the number of consecutive copied sheets, and stopping time from previous copying action, and toner concentration is controlled based on the output value Vt after correction and a toner concentration control reference value Vref.