A developer cartridge detachably mounted to a device main body includes a tubular cartridge main body that houses a developer and an extending portion that communicates with the cartridge main body, which extends from an end portion in a longitudinal direction of the cartridge main body. An outside shape of a section of the extending portion, along a direction that is orthogonal to the longitudinal direction, is smaller than that of the cartridge main body. A cartridge-side opening portion is provided at a side wall of the extending portion, and a cartridge-side shutter is provided at an outside of the side wall of the extending portion, which moves in a direction intersecting the longitudinal direction along the side wall to thereby open and close the cartridge-side opening portion.