PURPOSE: To obtain a modified resin excellent in weather resistance, rust prevention, adhesiveness, etc., and suitable as a vehicle of paints, etc., by modifying a chlorosulfonated polyolefin resin by reacting it with a fatty acid-modified resin or an unsaturated compound.

CONSTITUTION: The titled modified resin is prepared by mixing and reacting, with heating, a chlorosulfonated polyolefin resin with a fatty acid-modified resin or an unsaturated compound. As the fatty acid-modified resins, there can be used an alkyl resin, an epoxy resin, a phenolic resin and the like, which are modified with an oil containing conjugated dienes, conjugated trienes, conjugated acids or the like and containing a fatty acids of % conjugation ≥30% (e.g., tung oil, conjugated linseed oil). As the unsaturated compounds, there can be used vinyl comounds (e.g., styrene, acrylonitrile), vinylidene compounds (e.g., vinylidene chloride) and unsaturated carboxylic acid compounds (e.g., acrylic acid), etc.