It is aimed to provide a developer cartridge provided with a detection gear having a novel structure. A first gear (transmission gear 400) includes a small-diameter gear portion 450 and a large-diameter gear portion 440 having a diameter larger than a diameter of the small-diameter gear portion 450. The second gear (detection gear 300) includes: a first columnar portion centered on a second axis CL2; a second columnar portion having a smaller diameter smaller than the first columnar portion; a first engagement portion (first gear portion 332) extending along a portion of a peripheral surface of the first columnar portion and engageable with the small-diameter gear portion 450; a second engagement portion (second gear portion 352) extending along a portion of a peripheral surface of the second columnar portion and positioned closer to a housing than the first engagement portion to the housing in an axial direction and engageable with the large-diameter gear portion 440; and a protrusion (detection protrusion 301) protruding in the axial direction and rotatable together with the first engagement portion and the second engagement portion. The second engagement portion is engageable with the large-diameter gear portion 440 after the first engagement portion is engaged with the small-diameter gear portion 450.