An image forming apparatus having a heating device that is configured to heat an image bearing member so that the detection of the surface potential of the image bearing member can be controlled with greater stability. The image forming apparatus utilizes a surface potential detector and a temperature detecting member placed in positions at a precise distance relative to an exposure portion of the image bearing member, such that the stability of the surface potential detection can be improved. When a surface potential of the image bearing member is detected, it is possible to reduce the amount of change in the potential caused by carriers generated immediately after an exposure operation, and to reduce variations in detection of the surface potential caused by variations in the temperature, so that the surface potential can be controlled with greater stability.