In a case where a qualified cartridge is not mounted, a toner supply controller stops driving of a toner supply motor. A controller calculates the toner density in a toner tank in accordance with a detection signal of a TC sensor to determine whether the toner density is within an allowable range. In a case where the toner density is out of the allowable range, the toner supply controller allows the toner supply motor to start driving. After that, in a case where the toner density in the toner tank becomes high, a cartridge mounting status determining section determines that a non-qualified cartridge is mounted. On the other hand, in a case where the toner density in the toner tank does not change or becomes lower, the cartridge mounting status determining section determines that the cartridge is not yet mounted.