An example operation method of an image forming apparatus includes obtaining, from a sensor, an output signal corresponding to a degree of shrinkage of a transporting member that transports a developing agent in a toner cartridge, as the transporting member rotates, determining a state of the toner cartridge based on a characteristic of the output signal, and executing a recovery mode that increases a driving force delivered to the transporting member when determining that the state of the toner cartridge is an abnormal state in which toner is not normally supplied to a developing device.