Developing device reducing toner pressure on supply roller and image forming apparatus using same

A tandem-type color laser printer is provided with process units for each color which are arranged horizontally and in parallel with each other. In a developing unit in each process unit, a supply roller and a developing roller are disposed below a toner chamber, and a supply roller upper-side wall portion that covers the supply roller from above is provided between the toner chamber and the supply roller. This structure prevents the weight of toner stored in the toner chamber from directly acting on the supply roller, enables the toner between the supply roller upper-side wall portion and the supply roller to flow with the rotation of the supply roller, and secures preferable circulation of the toner.