A magnetic seal mounting method for mounting a magnetic seal member to a developing frame which has a magnetic seal mounting portion extended in a direction crossing with a longitudinal direction of a developing roller mounting portion for mounting a developing roller at each of one and the other longitudinal ends of the developing roller mounting portion, and a supporting member mounting portion for mounting a supporting member for supporting a developing blade extended along the longitudinal direction of the developing roller mounting portion, the method comprising the steps of: (a) engaging the magnetic seal member to the magnetic seal mounting portion while an elastic member between the magnetic seal mounting portion and the magnetic seal member is deformed, when the magnetic seal member is mounted to the magnetic seal mounting portion; (b) mounting the supporting member to a developing frame so that position of the magnetic seal member is regulated by the supporting member for supporting the developing blade to prevent disengagement of the magnetic seal member mounted to the magnetic seal mounting portion therefrom, thus mounting the developing blade; and (c) mounting the developing roller to the developing roller mounting portion provided in an including while flexing the developing blade after the developing blade is mounted to the developing frame.