The blade back seal, which integrally comprises the side portions extending in the vertical direction at the longitudinal end portions of the leaf spring member and the connecting portion for connecting the side portions at the longitudinal center portion of the leaf spring member, is interposed between the leaf spring member of the blade unit and the front attachment portion of the blade attachment member. At the opening of the case, the upper seal is interposed between the plate-like upper attachment portion, which extends in the longitudinal direction of the opening and has a width in the anteroposterior direction, and the extended portion of the bend preventing member of the blade unit, for sealing the space therebetween.