A developer supply cartridge detachably mountable to a main assembly of an image forming apparatus, includes a developer accommodating portion of accommodating a developer; a discharging opening for discharging the developer accommodated in the developer accommodating portion to a developer receiving opening of a developer receiving cartridge provided with a developing device for developing an electrostatic image formed on an image bearing member with the developer, the developer receiving cartridge being detachably mountable to the main assembly of the image forming apparatus; a shutter member movable between an opening position for opening the discharging opening and a closing position for closing the discharging opening; and an engaging portion for engagement with the developer receiving cartridge so as to receive, from the developer receiving cartridge, a force for moving the shutter member from the closing position to the opening position, in interrelation with relative movement between the developer supply cartridge and the developer receiving cartridge toward each other in a mounting-and-demounting direction of the developer supply cartridge relative to the main assembly of the image forming apparatus.