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 bias voltage to the development device; a calculating section which calculates a surface potential of the photosensitive drum based on a development current flowing in the development device; and an estimating section which estimates a fogging toner amount. The fogging toner amount is an amount of toner moved due to fogging. When the fogging toner amount estimated by the estimating section is less than a prescribed threshold, the calculating section calculates a value of the bias voltage at which the development current stops flowing as the surface potential.