A masking member contains parallel through-holes, each of the through-holes contains a tilted wall structure; an upper end of the tilted wall structure of one of the through-holes abuts on an upper end of the tilted wall structure of an adjacent one of the through-holes thereby forming a knife-edge ridge at the upper ends. The masking member may in contact with a substrate. Formation in quantity of various different populations of a substance being studied with multiple combinations of distribution form and distribution density may be conducted by dripping a suspension of a single concentration of the substance onto the masking member.