A cellulosic polymer wherein at least a part of the hydroxyl groups of the cellulosic polymer is modified as represented by the following general formula [I]: ##STR1## wherein R.sup.1 and R.sup.2 may be the same or different and each represents a hydrogen atom or a C.sub.1 to C.sub.5 alkyl group, X is a strong negative atomic group, and Y is ##STR2## wherein R.sup.3 and R.sup.4 may be the same or different and each represents a member selected from the group consisting of a hydrogen atom, a C.sub.1 to C.sub.5 alkyl group and a C.sub.3 to C.sub.6 cycloalkyl group, or ##STR3## Y is wherein R.sup.5, R.sup.6 and R.sup.7 may be the same or different and each represents a member selected from the group consisting of a hydrogen atom, a C.sub.1 to C.sub.5 alkyl group, a C.sub.3 to C.sub.6 cycloalkyl group and a C.sub.6 to C.sub.10 aryl group, and n is from 1 to 10. Using the cellulosic polymer, a blood-treating device is assembled.