A developer cartridge includes a first developer wall formed with a convex part and a second developer wall disposed in confronting relation with the first developer wall. The convex part of the first developer wall is insertable into an insertion portion of an image-bearing member cartridge when the developer cartridge is mounted on the image-bearing member cartridge. When a plurality of the developer cartridges are stacked one on the other with the first developer wall being downside with respect to the second developer wall facing upward, the first developer engagement part in one developer cartridge engages the second developer engagement part in another developer cartridge disposed just below the one developer cartridge.