A developer cartridge (60) capable of being fitted to the toner replenishing opening of a developer storage hopper (50) has a sealing member (63) for sealing the toner therein that is bonded with an adhesive (62) etc., to the rim of its replenishing opening (61) to be opposed to the opening of the developer storage hopper (50). The bottom face of this developer cartridge (60) which constitutes the bottom for supporting the toner charged therein is inclined upwards in the stripping direction of the sealing member (63), whereby the charged amount of the toner at the stripping start of the sealing member (63) is regulated when the sealing member (63) is stripped so that the toner can be uniformly charged across the full length of the developer storage hopper (50) with respect to the stripping direction of the sealing member (63).