Abstract: The present invention provides a processing composition which is used for lithographic printing plate where silver image is used as ink receptive area for enhancing ink receptivity of the silver image. This composition contains an organic compound having two or more mercapto groups or thion groups and, in combination therewith, an aromatic sulfonic acid formalin condensate, an anionic surfactant having a polyoxyethylene group, an aromatic carboxylic acid, a hydrosulfite compound, a water-soluble polymer having a carboxyl group and having a number-average molecular weight of about 20,000 or less, a hydroxyaryl fatty acid, or an organic disulfide compound. If this composition additionally contains an oxidizing agent, it can further improve printing endurance of the printing plate.