Abstract: Disclosed is a novel diazo resin which has at least one repeating unit represented by the following general formula (I): ##STR1## wherein R.sup.2 represents a hydrogen atom, an alkyl group or an alkoxy group, a hydroxyl group, a carboxy ester group or a carboxyl group: R.sup.1 represents a carboxyl group or a group having at least one carboxyl group; R.sup.3 and R.sup.4 each represents a hydrogen atom or an alkyl group or an alkoxy group; X.sup.31 represents an anion; and Y represents --NH--, --O-- or --S--. The diazo resin is useful for a presensitized plate for use in making a lithographic printing plate when it is incorporated into a light-sensitive layer or an underlayer on a substrate. According to the present invention, a presensitized plate using the diazo resin has excellent properties such as high sensitivity, good adhesion between the substrate and the light-sensitive layer and can thus provide a lithographic printing plate having a high printing durability and free of background contamination.