Desensitizing gum for planographic printing plates

In a gumming-up process comprising applying to an imagewise exposed and developed presensitized plate, a desensitizing gum comprising water having dissolved therein 5 to 40 wt. % of a water-soluble high molecular compound, the improvement characterized in that said water-soluble high molecular compound comprises about 0.1 to 40 wt. %, based on the total weight of the gum, of a water-soluble carboxy alkylated starch wherein the carboxy alkyl group number of said carboxy alkylated starch is from 0.03 to 0.5, and wherein about 90 wt. % or more of the starch skeleton of said alkylated starch is of amylopectin type.