An image forming apparatus capable of reducing the number of toner concentration detection errors caused by variation in the volume of a toner includes a reverse vane for transferring developer in an opposite direction and protruding from the rotary shaft member in a region where a lower side faces a bottom wall of a first transfer chamber and two lateral sides that are orthogonal to the rotary axis direction face respective side walls of the first transfer chamber, and a toner concentration detection sensor is disposed to detect the toner concentration of the developer as the developer is transferred between the reverse transfer vane and a location of the screw vane adjacent to the reverse transfer vane.