An image forming apparatus includes a developer container that holds a two-component developer, a toner concentration detector that detects toner concentration in the developer, a toner supplying unit configured to supply new toner to a developing unit, a toner supply controlling unit that compares an output of the toner concentration detector and a toner concentration reference value Vtref to control the toner supplying unit. The image forming apparatus detects an output of the toner concentration sensor with respect to a plurality of linear velocities, and calculates linear velocity shift amounts .DELTA.Vt1=(Vt1-Vt0) and .DELTA.Vt2=(Vt2-Vt0) to be reflected in a toner supply control parameter.