An image forming apparatus includes an alternating transformer that splits an alternating voltage into a first alternating voltage and a second alternating voltage, a developer bias application unit, an alternating voltage shutoff unit, and a discharge starting voltage measurement unit. The developer bias application unit applies a pulsed sleeve roller voltage composed of a first alternating voltage superimposed on a first direct voltage to a sleeve roller and also applies a second alternating voltage and a second direct voltage separately to a magnetic roller in order to apply a pulsed magnetic roller voltage composed of the second alternating voltage superimposed on the second direct voltage. The alternating voltage shutoff unit stops applying the second alternating voltage to the magnetic roller. The discharge starting voltage measurement unit increases the alternating voltage of the alternating transformer to measure the discharge starting voltage between the sleeve roller and magnetic roller.