An object of the present invention is to provide a surface treatment agent that is excellent in antithrombogenicity and biocompatibility and has higher hydrophilicity compared to conventional medical materials. The present invention is an antithrombogenic material containing a (meth)acrylate copolymer consisting of a hydrophobic (meth)acrylate and a hydrophilic (meth)acrylate, in which the hydrophobic (meth)acrylate is a silicone (meth)acrylate and/or an alkyl (meth)acrylate, the (meth)acrylate copolymer having a residual monomer content of 4,000 ppm or less, a reduced viscosity (ηsp/c) of 0.18 dl/g or more and 3.00 dl/g or less, and being water-insoluble, and being a viscous liquid at room temperature.