An image forming apparatus includes: a development device which develops an electrostatic latent image formed on a photosensitive drum into a toner image; a charger which charges the photosensitive drum; a development power supply which applies a prescribed bias voltage to the development device; and a calculating section which calculates a surface potential of the photosensitive drum based on a development current flowing in the development device. The calculating section calculates, as the surface potential, a bias voltage at which a non-charging development current that flows in the development device in an uncharged state in which the charger has not charged the photosensitive drum is equal to a charging development current that flows in the development device in a charged state in which the charger has charged the photosensitive drum.