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 Ua1, Ua2, Ub1, Ub2 and Ub3, and An- are as defined in the present specification and the claims.
