The image forming apparatus includes a developing unit that is detachable and contains a developer, a detection member that includes an electrode to be detected and is rotatable around a rotation axis in the developing unit, an agitator that moves around the rotation axis in the developing unit; an electrostatic capacitance sensor electrode provided on an exterior of the developing unit, an electrostatic capacitance sensor that detects electrostatic capacitance between the electrode to be detected and the electrostatic capacitance sensor electrode, and outputs data on the detected electrostatic capacitance, and a CPU that determines an amount of developer in the developing unit based on the data output from the electrostatic capacitance sensor.