An image forming apparatus includes an image bearing member and a charging device configured to charge the image bearing member. The charging device includes a magnetic particle carrier and a magnetic particle regulating member configured to regulate magnetic particles carried by the magnetic particle carrier. The charging device causes the magnetic particles carried by the magnetic particle carrier to contact the image bearing member, and applies a voltage to the magnetic particle carrier to charge the image bearing member. An electrode has a contact area via which the electrode can contact magnetic particles stored in a particle pool defined by the magnetic particle carrier and the magnetic particle regulating member. The contact area is variable according to an amount of the magnetic particles stored in the particle pool. A current detection device detects a value of current flowing from the magnetic particle carrier to the electrode via the magnetic particles.