An image forming apparatus includes image forming units, a toner supply device, a common-unit detection device, an initial operation execution device, and a determination device. Each image forming unit is detachably provided in the image forming apparatus, and includes at least a development device containing toner. At least two of the image forming units are common image forming units having a common structure. When the common-unit detection device detects a common image forming unit at installation thereof, the initial operation execution device supplies a predetermined amount of toner to a development device of the common image forming unit and mixes to charge the toner by friction for a predetermined time period. Then, the. determination device detects whether the toner in the development device is charged to a predetermined level, and determines, based on the detection, whether to mix the toner one more time for another predetermined time.