A developing cartridge includes: a casing configured of a first frame and a second frame with a space formed therebetween; and a developer-carrying member that is supported in the first frame and that is configured to carry developer. The first frame includes a bottom wall and a pair of side walls erected from both sides of the bottom wall and has a box shape that is open on a side opposite the bottom wall. The second frame is mounted over the pair of side walls so as to cover the open side of the first frame and has a partition extending into the space formed in the casing. The partition contacts the bottom wall of the first frame and spans between the pair of side walls of the first frame, thereby separating the space within the casing into a first chamber on a side near the developer-carrying member and a second chamber on a side far from the developer-carrying member. Developer is accommodated in the first chamber only.