An apparatus for processing an imagewise exposed pre-sensitized lithographic printing plate which entails uniformly applying a desired amount of developer, that has not been substantially used to the surface of the printing plate by a developer supply member comprised of two plates and having a slit at the lowermost portion thereof which contacts the surface of the printing plate, and then transferring the plate containing the developer to a developer storage tank, immersing the plate in the developer storage tank, and removing the developer from the plate.