To provide a developing device and the carrying method of developer in which the reduction in a solid density after supplying new toner is prevented.

The device is provided with a developing roller 1, which carries developer having a nonmagnetic single component, a supplying roller 2, which supplies the developer located in a freely exchangeable toner cartridge to the roller 1, more than one bias voltage applying means which apply a developing bias voltage Vb to the roller 1 and apply a bias voltage Vsp to the roller 2 and a control means, which at least varies a potential difference Vsp-Vb. A stable carrying of the toner is accomplished by increasing the potential difference Vsp-Vb during a flat picture outputting after the exchange of a toner cartridge through the use of the control means.