A toner supply device for supplying toner to a developing unit includes: a toner cartridge; and a toner feed device having a casing for reserving toner and a toner agitator shaft for agitating and conveying the toner. The toner feed device includes a toner input port for allowing entrance of toner into the casing and a toner feed port for feeding toner to the developing unit, the toner input port and toner feed port being arranged on the outer side of the developing roller with respect to the direction of the developing roller's axis of the developing unit, and the toner agitator shaft conveys and supplies the toner from the toner input port along the direction of the developing roller's axis to the toner feed port.