In accordance with an embodiment, a developing agent cartridge for supplying developing agent and toner to a developing device comprises a cartridge main body, in which there is a first room and a second room and the boundary of the two rooms is closed by a closure member, configured to respectively fill the toner and the developing agent into the first room and the second room; a supply section configured to supply the infill in the second room to the developing device while stirring it; and an opening section configured to open the closure member to enable the toner to be moved to the second room after the developing agent filled into the second room is supplied to the developing device.