A cartridge includes a casing, a developing roller, and an agitator. The casing is configured to accommodate developing agent. The casing has a first wall and a second wall disposed away from the first wall in a first direction. The agitator includes a rotational shaft and an agitating blade. The rotational shaft extends in the first direction and has a first end portion. The first end portion defines a circumferential surface and is supported to the first wall. The rotational shaft is configured to rotate in a rotating direction about a rotational center. The first wall has a concave portion having a plurality of protruding portions extending toward the circumferential surface and arranged at intervals from each other in the rotating direction. The agitating blade is positioned between the first end portion and the second end portion in the first direction and extending in the radial direction.