A side seal for preventing the leakage of toner out of a housing frame is disposed at each end portion in the axial direction of a developing roller within the housing frame, and is configured of a sponge seal affixed to the housing frame and a sliding sealing element that partially covers the sponge seal. Each sliding sealing element is affixed by double-faced adhesive tapes to an upper attachment portion of a plate spring above the upper edge of the sponge seal and to a lower attachment portion of the housing frame below the lower edge of the sponge seal, without being affixed to the sponge seal, per se.