A developing device includes a developer container, a developer carrier, and a stirring/conveying member. The developer container stores developer containing toner. The developer carrier carries on its surface the toner in the developer container. The stirring/conveying member includes a rotary shaft rotatably supported on the developer container and a stirring blade formed around the outer circumferential face of the rotary shaft and stirs and conveys the developer in the developer container. The stirring/conveying member to be used can be selected from a plurality of types of stirring/conveying members with rotary shafts with different shaft diameters.