PURPOSE: To obtain an alkali-developable photosensitive composition suitable for use in the production of a photosensitive lithographic printing plate having high printing resistance and giving a strong image at the highlight part by combining modified polyvinyl acetal resin with specified diazo resin.
CONSTITUTION: This photosensitive composition contains modified polyvinyl acetal resin obtd. by introducing substituents having acidic hydrogen atoms into the residual hydroxyl groups in polyvinyl acetal resin so that the acid content is regulated to 0.1W6 milliequiv. per 1g resin, and diazo resin represented by formula I (where R 1 is H, carboxy or the like, R 2 is H, 1W3C alkyl or phenyl, R 3 is H or 1W3C alkoxy, R 4 is 6W18C alkyl and n is an integer of ≥2). When the photosensitive composition is used, a photosensitive lithographic printing plate having further increased printing resistance and capable of giving a strong highlight image is obtd.