A surface treating agent for use as an undercoat when painting a shaped article of a polyolefin comprises a solution in an organic solvent of a propylene-ethylene copolymer having a monoalkyl ester of a monoolefinic dicarboxylic acid, such as monomethyl maleste, grafted thereto, the propylene-ehylene copolymer having a propylene content of 50 to 75 mole %, and a crystallinity determined by an X-ray diffraction method, of 2 to 20%, the esterification degree of the alkyl ester of the monoolefinic dicarboxylic acid being 45 to 100% if the esterification degree is taken as 200% when all of the carboxyl groups of the dicarboxylic acid are esterified, the amount of the monoalkyl ester grafted being 0.5 to 15% by weight, the grafted propylene-ethylene copolymer having an intrinsic viscosity [η] of at least 0.3 and being dissolved in an amount of 10 to 100 kg per cubic meter of the organic solvent. The surface treating agent when used as an undercoat greatly improves the adhesion of a paint to a polyolefin shaped article, and has excellent storage stability.