A blood anticoagulant material is provided which has high expandability, reduced influence on living bodies, and can efficiently prevent blood coagulation. A multiblock copolymer having at least a structural unit represented by Formula (1), a structural unit represented by Formula (2), and a structural unit represented by Formula (3) is used: ##STR00001## wherein, R.sup.1, R.sup.2, and R.sup.3 in Formulas (1) to (3) each independently represent a divalent organic group, and m and n each independently represent an integer of 2 to 5000.