An image forming apparatus, includes: an image carrier that carries a developer image by a charge layer provided on a surface of the image carrier; a charger that charges the image carrier; a feeder that feeds the charger with a charge; a total charge amount detector that detects the total amount of the charge output by the feeder; an unsupplied charge amount calculator that calculates the amount of the charge output by the feeder but not supplied to the charger; and a layer thickness calculator that calculates the thickness of the charge layer based on the unsupplied charge amount calculated by the unsupplied charge amount calculator and the total charge amount detected by the total charge amount detector.