A developing unit detachably mountable to a main assembly of an electrophotographic image forming apparatus, includes a developing member for developing an electrostatic latent image formed on an electrophotographic photosensitive member, a developer accommodating portion accommodating developer to be used by the developing member and having a supply opening for supplying the developer to the developing member, a mover sealing member movable between supply-opening sealing and unsealing positions, a mover for moving the sealing member from the sealing to the unsealing position by receiving a driving force transmitted from the main assembly, and a memory for storing sealing information indicative of a sealed state of the supply opening. When the main assembly detects the sealing information upon mounting of the developing unit, the driving force is transmitted to the mover to move the sealing member from the sealing to the unsealing position, and then, the memory stores the unsealing information.