A biomolecule-compatible highly branched polymer obtained by polymerizing a monomer having an alkylene oxide and two or more radically polymerizable double bonds in a molecule in presence of a polymerization initiator in 5 mol % or more and 200 mol % or less relative to the moles of monomer, wherein molecular terminals of biomolecule-compatible highly branched polymer have one biomolecular site of at least one pair from the group of combination pairs of biotin and avidin, an antigen and antibody, polynucleotide and polynucleotide having complementary base sequence thereof, cDNA and mRNA, enzyme and substrate, an enzyme and product, an enzyme and competitive inhibitor, an enzyme (binding site) and coenzyme, an enzyme (binding site) and triazine dye, protease and protease inhibitor, Fc site and protein A, Fc site and protein G, lectin and sugar, hormone receptor and hormone, DNA and DNA binding protein, heparin and fibronectin, and heparin and laminin.