Image forming apparatus controlling surface potential of image bearing member

A control portion executes: a step of calculating a surface potential of an image bearing member charged by a charging member, to which a predetermined voltage is applied, and exposed to light by a predetermined exposure amount on the basis of a discharge start voltage between a measurement member and the image bearing member and calculating a correction value from a difference between a target value and calculated surface potential of the image bearing member; a step of correcting calculated surface potential of the image bearing member on the basis of the correction value; and a step of controlling at least one of the voltage applied to the charging member and an exposure amount of the exposure apparatus on the basis of the corrected surface potential of the image bearing member so that the surface potential of the image bearing member reaches the target value.