The present invention relates to a filter built into a blood purification column, and aims to dramatically improve the removal of air that gets into the column and to prevent blood activation due to air remaining inside of the column and decreased absorption performance due to decreased contact surface between blood and an absorbent. This blood purification column has an absorbent and a casing, both ends of which are open ends, and the absorbent is housed inside of the casing. Of the two ends of the casing, one is the blood inflow end and the other is the blood outflow end. A filter is arranged on the blood inflow end and/or the blood outflow end of the casing, and this filter fulfils the following conditions. (1) The opening ratio of the filter is 5-80% (2) The equivalent diameter of the openings in the filter is 1-5000µm (3) The ratio of the equivalent diameter of the openings to the average circle equivalent diameter of the gaps in the absorbent is 45% or greater.
