<p>It is intended to provide a practically applicable substrate for blood treatment for specifically capturing a target substance such as a cell from whole blood directly and a substrate for biological fluid treatment which can be industrially used at a low cost and with which a target substance can be separated directly from a biological fluid. According to the invention, the substrate for biological fluid treatment in which a recognition molecule having a selective binging ability to a target substance is immobilized on a surface of the substrate by a covalent bond, characterized in that the recognition molecule is a linear molecule containing a ligand region and an association domain and at least four or more recognition molecules form an associated structure is provided.</p>
