The image bearing member includes an image bearing member, a charge unit that charges the image bearing member, an exposure unit that exposes the image bearing member by emitting light thereto, a transfer unit that transfers an image formed on the image bearing member, a detection unit that detects a current flowing between the transfer unit and the image bearing member when a voltage is applied to the transfer unit, and a control unit that causes the charge unit to charge the image bearing member through application of a DC voltage, and calculates a surface potential of the image bearing member based on a detection result of the current when the voltage is applied to the transfer unit, after a light whose light amount is higher than a light amount of a light emitted to the image bearing member from the exposure unit is emitted to the image bearing member.