An image forming apparatus includes: a photoreceptor having a photosensitive layer formed on a surface; a charging device that electrically charges the surface of the photoreceptor through electric discharge between the charging device and the photoreceptor; and a hardware processor that: calculates a peak-to-peak voltage to be applied to the charging device, using a measured value of a relative dielectric constant of the charging device, the relative dielectric constant having been measured in advance; and controls a voltage to be applied to the charging device, to apply the peak-to-peak voltage calculated by the hardware processor to the charging device.