This is to provide a blood filter comprising a porous element; and a copolymer which exists in at least a part of a surface of the porous element, and which contains a recurring unit containing an organic group of the following formula (a) and a recurring unit containing an organic group of the following formula (b), wherein U a1 , U a2 , U b1 , U b2 and U b3 , and An - are as defined in the present specification and the claims.本發明為提供一種多孔質元件、及存在於該多孔質元件之表面的至少一部分的含有以下共聚物之血液過濾器,該共聚物為含有具有下述式(a)所示有機基之重複單位與具有下述式(b)所示有機基之重複單位者;(式中,Ua1、Ua2、Ub1、Ub2及Ub3、以及An-如說明書及申請專利範圍所記載)。