An image forming apparatus including a photoreceptor that rotates, a charging member provided in contact with or in close proximity of the photoreceptor to charge the photoreceptor, a charge amount detecting portion that accumulates current flowing across the photoreceptor to obtain a charge amount of the photoreceptor, while the current is being applied from the charging member until a voltage on a surface of the photoreceptor substantially corresponds to the voltage applied by the charging member, and a controller that calculates a film thickness of the photoreceptor based on the charge amount.