PURPOSE: To provide a satisfactory resolving power in a wider range of molecular weight of protein or the like, by giving a gradient in the layer thickness to a gel medium layer comprising a polyacrylic amide water gel formed between a planar support and a planar cover sheet.

CONSTITUTION: A spacer plate 1 with a gradually varying thickness of a section along the direction of electrophoresis is fixed at both rims of a support made of polyethylene terephthalate (PET) with the surface thereof subjected to an ultraviolet irradiation treatment. Moreover, the work is covered with a cover sheet made of PET along the spacer plate 1 and the outside of the cover sheet is fixed with an aluminum plate so as to respond to changes in the thickness of the spacer plate 1, whereby a mold is formed. An aqueous solution for forming acrylic amide gels is run into the mold and irradiated with light from a high pressure mercury lamp to cause a crosslinked polymerization. This can produce a gel medium membrane having a satisfactory resolving power in a wide range of molecular weight of protein or the like.