The sponge sealing member of the side seal is arranged on the roller opposed face of the seal arrangement portion so that the lower end face at the upstream side in the rotating direction of the developing roller is positioned to have the same face as the back end face of the seal arrangement portion and the upper end face at the downstream side is connected with the lower end face of the side portion of the blade back seal. The felt sealing member is arranged so that the upper end portion at the downstream side in the rotating direction of the developing roller is formed astride the lower end portion of the leaf spring member and the sponge sealing member, extends along the sponge sealing member, through the back end face of the seal arrangement portion and around to the lower face of the seal arrangement portion.