A developing cartridge comprising: a casing configured to accommodate toner therein, the casing including an inner wall constituting a groove and anothergroove recessed inward of the casing, the groove and the another groove being arrayed in a first direction; a developing roller rotatable about a rotational axis extending in the first direction, the developing roller having an outer circumferential surface; a blade configured to regulate a thickness of a layer of toner formed on the outer circumferential surface of the developing roller, the blade extending in a second direction toward the outer circumferential surface of the developing roller and having one end portion and another end portion in the second direction, the another end portion of the blade being positioned away from the one end portion of the blade in the second direction, the another end portion of the blade being positioned farther from the outer circumferential surface of the developing roller than the one end portion of the blade is from the outer circumferential surface of the developing roller; and a blade holder supporting the another end portion of the blade, the blade holder including: a first plate extending in the second direction and supporting the another end portion of the blade; and a second plate extending from the first plate in a third direction crossing the second direction, the second plate having a distal end portion positioned within the groove and the another groove, and characterized in that the groove and the another groove are arrayed in the first direction with a prescribed interval therebetween, the distal end portion of the second plate is positioned within the groove and the another groove with a gap between the distal end portion of the second plate and the inner wall, the developing cartridge further comprises a thread fixing the second plate to the casing, and the thread has a portion positioned between the groove and the another groove in the first direction. a casing configured to accommodate toner therein, the casing including an inner wall constituting a groove and anothergroove recessed inward of the casing, the groove and the another groove being arrayed in a first direction; a developing roller rotatable about a rotational axis extending in the first direction, the developing roller having an outer circumferential surface; a blade configured to regulate a thickness of a layer of toner formed on the outer circumferential surface of the developing roller, the blade extending in a second direction toward the outer circumferential surface of the developing roller and having one end portion and another end portion in the second direction, the another end portion of the blade being positioned away from the one end portion of the blade in the second direction, the another end portion of the blade being positioned farther from the outer circumferential surface of the developing roller than the one end portion of the blade is from the outer circumferential surface of the developing roller; and a blade holder supporting the another end portion of the blade, the blade holder including: a first plate extending in the second direction and supporting the another end portion of the blade; and a second plate extending from the first plate in a third direction crossing the second direction, the second plate having a distal end portion positioned within the groove and the another groove, and a first plate extending in the second direction and supporting the another end portion of the blade; and a second plate extending from the first plate in a third direction crossing the second direction, the second plate having a distal end portion positioned within the groove and the another groove, and characterized in that the groove and the another groove are arrayed in the first direction with a prescribed interval therebetween, the distal end portion of the second plate is positioned within the groove and the another groove with a gap between the distal end portion of the second plate and the inner wall, the developing cartridge further comprises a thread fixing the second plate to the casing, and the thread has a portion positioned between the groove and the another groove in the first direction.