A surface treatment agent for hydraulic transfer to perform hydraulic transfer decoration with high adhesion onto an olefin-type substrate with poor adhesion. They are provided by a surface treatment agent for hydraulic transfer to a polyolefin-type substrate to be used in a hydraulic transfer method for forming a decorative layer, by coating an activator to a dried print pattern on a water-soluble film to recover adhesion of said print pattern, and then by hydraulic transferring said print pattern onto the surface of a surface treatment layer of the polyolefin-type substrate treated with the surface treatment agent, characterized in that said surface treatment agent for hydraulic transfer is consisted of a resin composition including a modified polyolefin (a), a polyfunctional isocyanate (b) and a solvent (c), and a blending ratio of (b)/(a) is 0.005 to 0.5, in weight ratio.