Abstract: A laser plate-making apparatus includes a moistureproof paper feed magazine for feeding plate materials one by one, each having a photosensitive layer, a charger for charging the plate material with an electric charge, an exposing unit for exposing the plate material charged with the electric charge, using a laser beam modulated in accordance with a plate-making image to partially neutralize the electric charge and form an electrostatic latent image, a developing unit for developing the electrostatic latent image by applying a developing solution to the plate material, the developing unit being located at the lowest position in a convey path, and including an arcuated pair of electrodes through which the plate material passes and to which a predetermined voltage is applied, a developing solution concentration sensor having detachable special cells and using a light-emitting diode, and a squeezing roller pair for squeezing out an excess developing solution from the developed plate material, the developing solu