In an image forming apparatus, a seal member is a non-conductive, flexible member supported by a developing device. The seal member is formed to project from an edge part of the developing device that faces an image carrying member, along a longitudinal direction of the image carrying member. The seal member fills a part of a gap between the developing device and an outer circumferential surface of the image carrying member. An electrode film is adhered to a surface of the seal member. The electrode film is a conductive film. A potential detecting device detects a potential of the electrode film. A charging controller controls a charging voltage based on a result of comparison between a predetermined reference potential and the potential detected by the potential detecting device.