Image forming apparatus that accommodates cartridge with sufficient amount of developer

An image forming apparatus comprising:

• a main casing comprising a first wall and a second wall extending in a first direction and opposing each other in a second direction orthogonal to the first direction, the main casing further including a main-casing positioning plate disposed inward of the first wall in the second direction and positioned closer to the first wall than to the second wall in the second direction;
• a supporting frame configured to move relative to the main casing in the first direction between an internal position accommodated in the main casing and an external position withdrawn from the main casing, the main-casing positioning plate being positioned outward of the supporting frame in the second direction when the supporting frame is at the internal position;
• an image carrier configured to carry an electrostatic latent image thereon, the main-casing positioning plate being configured to provide positioning of the image carrier relative to the main casing;
• and
• a developer container configured to be supported in the supporting frame and configured to store developer therein, the developer container comprising:
• a first chamber provided in the supporting frame;
• a second chamber configured to store developer therein and disposed opposite to the first chamber with respect to the main-casing positioning plate when the supporting frame is at the internal position;
• a third chamber configured to establish communication between the first chamber and the second chamber;
• and
• a conveying portion configured to convey the developer stored in the second chamber to the first chamber via the third chamber.