Developing unit having guide that stably supports toner cartridge

A process cartridge detachably attachable to a main casing of an image forming apparatus, the process cartridge comprising:

• a photosensitive drum;
• a developing roller configured to supply toner to the photosensitive drum;
• a cartridge accommodating part;
• and
• a toner cartridge configured to store toner therein and detachably attachable to the cartridge accommodating part, the toner cartridge comprising:
• a first casing defining an internal space therein and having a supply hole for allowing the toner to pass therethrough toward the developing roller, the first casing including a shutter movable between an open position where the supply hole is open and a closed position wherein the supply hole is closed,
• the toner cartridge being pivotable about the first casing relative to the cartridge accommodating part between a first position where the shutter is in the open position and attachment of the process cartridge to the main casing is allowed and a second position where the shutter is in the closed position and attachment of the process cartridge to the main casing is not allowed.