Toner supplying method, toner supplying device, developing device, and image forming apparatus

A toner supplying device includes: a toner density sensor for detecting a density of toner contained in a developer tank; a toner hopper for supplying toner to the developer tank; a toner cartridge for supplying toner to the toner hopper; and a control section, when a request to supply toner to the developer tank is made in accordance with a signal detected by the toner density sensor, for controlling a hopper driving motor and a cartridge driving motor so that (i) the toner hopper supplies toner to the developer tank and (ii) the toner cartridge supplies toner to the toner hopper. Thus, in a structure having first and second toner supplying sections disposed on top of each other, toner can be stably supplied to a developer tank.