A light-sensitive composition which is developable with an aqueous alkaline solution, comprising (a) at least one light-sensitive resin obtained by adding an active mercaptocarboxylic acid to a part of carbon-carbon unsaturated bonds of a polymer having at least two photodimerizable unsaturated double bonds adjacent to an aromatic nucleus in the main chain thereof and (b) optionally at least one negative-working diazo resin. The light-sensitive composition makes it possible to prepare a light-sensitive plate capable of developing with an aqueous alkaline solution without forming insoluble substances in the developer during the development and the adhesive force between the substrate and the light-sensitive layer of the plate is increased, which in turn substantially improves the printing durability of the lithographic printing plate as a final product.