A developer cartridge comprising: a housing extending in a first direction and configured to contain toner, the housing having a first end in the first direction, and a second end located away from the first end in the first direction; a developing roller rotatable about a roller axis extending in a second direction different from the first direction, the developing roller being located closer to the first end than to the second end in the first direction, the developing roller having an uncovered surface located at a first end of the developing roller in a third direction and exposed outside the housing, and a covered surface located at a second end of the developing roller in the third direction and located inside the housing; a first boss extending in the second direction and movable together with the housing, the first boss being located at a first outer surface of the housing in the second direction, the first boss being configured to bear a weight of the developer cartridge when the first boss is supported by a support surface of a first side plate of a drum unit, the first boss being located away from the developing roller in the first direction and the third direction; and a first pressure receiving surface located farther away from the developing roller than the first boss is from the developing roller in the third direction, the first pressure receiving surface being configured to receive a pressing force from a pressing member of the first side plate of the drum unit, wherein the first boss is located away from the developing roller in the first direction by a first distance, and wherein the first pressure receiving surface is located away from the developing roller in the first direction by the first distance or by a second distance smaller than the first distance, and characterized in that the first pressure receiving surface receives from the pressing member of the first side plate of the drum unit the pressing force directed from the covered surface toward the uncovered surface in the third direction to move the developing roller toward a photosensitive drum of the drum unit when the developer cartridge is attached to the drum unit. a housing extending in a first direction and configured to contain toner, the housing having a first end in the first direction, and a second end located away from the first end in the first direction; a developing roller rotatable about a roller axis extending in a second direction different from the first direction, the developing roller being located closer to the first end than to the second end in the first direction, the developing roller having an uncovered surface located at a first end of the developing roller in a third direction and exposed outside the housing, and a covered surface located at a second end of the developing roller in the third direction and located inside the housing; a first boss extending in the second direction and movable together with the housing, the first boss being located at a first outer surface of the housing in the second direction, the first boss being configured to bear a weight of the developer cartridge when the first boss is supported by a support surface of a first side plate of a drum unit, the first boss being located away from the developing roller in the first direction and the third direction; and a first pressure receiving surface located farther away from the developing roller than the first boss is from the developing roller in the third direction, the first pressure receiving surface being configured to receive a pressing force from a pressing member of the first side plate of the drum unit, wherein the first boss is located away from the developing roller in the first direction by a first distance, and wherein the first pressure receiving surface is located away from the developing roller in the first direction by the first distance or by a second distance smaller than the first distance, and characterized in that the first pressure receiving surface receives from the pressing member of the first side plate of the drum unit the pressing force directed from the covered surface toward the uncovered surface in the third direction to move the developing roller toward a photosensitive drum of the drum unit when the developer cartridge is attached to the drum unit.