Abstract: The present invention relates to a light-sensitive composition comprising a modified polyvinyl acetal resin having a substituent possessing an acid hydrogen atom introduced into remaining hydroxy groups of a polyvinyl acetal resin in an amount of from 0.1 to 6 milliequivalents as acid content per 1 gram resin, and a diazo resin represented by the formula (I) ##STR1## wherein R.sub.1 represents a hydrogen atom, or a substituted or unsubstituted alkyl, alkoxy, hydroxy, carboxyester or carboxy group; R.sub.2 represents a hydrogen atom, an alkyl group having 1 to 3 carbon atoms or a phenyl group; R.sub.3 represents a hydrogen atom or an alkoxy group having 1 to 3 carbon atoms; R.sub.4 represents an alkyl group having 6 to 18 carbon atoms; n represents an integer not smaller than 2.The presensitized lithographic printing plate using the light-sensitive composition of the present invention can be developed with an alkali developer and the highlight image thereof is tough.