A developer cartridge assembled to a developing device defining therein a developing chamber. The developer cartridge includes a first accommodating portion, and a first agitator. The first accommodating portion accommodates therein a developing agent and is positioned beside the developing device. The first accommodating portion includes a first wall part neighboring the developing device. The first wall part is formed with a first through-hole and a second through-hole. The first through-hole is open diagonally downward from the first accommodating portion to the developing device. The second through-hole is open diagonally upward from the first accommodating portion to the developing device. The first through-hole and the second through-hole allow communication between an interior of the first accommodating portion and the developing chamber. The first agitator is rotatably disposed in the first accommodating portion and in sliding contact with the first wall part in a vertical direction.