A developing roller includes a roller main body disposed to face, without contact, an outer circumferential surface of an image carrier. A resin coat layer has been formed on an outer circumferential surface of the roller main body, the resin coat layer being made of a resin material having electric conductivity. A product of resistance component Rs [Ω] and electrostatic capacitance component Cs [F] in AC impedance Z of the roller main body is in a range from 2.79×10−7 to 6.77×10−5, the AC impedance Z being obtained when an AC voltage of a predetermined frequency f is applied.