PURPOSE: To achieve an accurate measurement of percentage of fractioned protein, by limiting coefficient of electroosmosis of a fine porous film mainly composed of cellulose acetate within a specified range while an average pore diameter or a surface pore diameter thereof is limited within a specified range.

CONSTITUTION: A fine porous film mainly composed of cellulose acetate has a coefficient of electroosmosis within a range of 0W-3mm while an average pore diameter is 0.2W5μm and a surface pore diameter of a sample coated surface within a range of 10W30μm. Thus, an origin is separated from a protein fractioning zone completely and after electrophoresis, there is no coat residue at a coating point thereby producing a support with individually clear fractions.