An image forming apparatus which comprises a developing unit having a toner and a carrier therein, the apparatus includes an agitator to agitate the toner, a toner sensor to receive a control voltage and to generate an output voltage corresponding to a remaining amount of the toner in the developing unit, a pulse width modulator to supply the control voltage to the toner sensor corresponding to a pulse width modulation (PWM) signal and a controller to control the pulse width modulator to supply the control voltage to the toner sensor to adjust a duty ratio of the PWM signal and make the output voltage reach a target value. The controller determines whether the agitator is normal or abnormal based on the output voltage of the toner sensor.