An image forming apparatus includes a photosensitive member and a developer carrying member spaced from the photosensitive member by a gap. The developer carrying member carries a developer containing an externally added strontium titanate. A bias voltage is applied between the photosensitive member and the developer carrying member with a substantially rectangular waveform. This bias voltage forms an oscillating electric field therebetween. The electric field provided by the difference between a jump peak voltage of the bias voltage and the potential of the photosensitive member at a portion exposed to the light is not less than 4.1 V/micron. The electric field provided by a difference between the jump peak voltage and a potential of the photosensitive member at a portion not exposed to light is not more than 3.5 V/micron. The electric field provided by a difference between a returning peak voltage of the bias voltage and a potential of the photosensitive member at a portion not exposed to light is not more than 2.9 V/micron. A ratio of a portion of the voltage for the direction toward the photosensitive member is 20 to 50%.