Abstract: This invention is directed to an electrode for use in electroretinography. The electrode contains a thin membrane made of a hydrogel of high water content and a conductor connected to the thin membrane. The thin membrane is prepared by casting an aqueous polyvinyl alcohol solution into a mold, freezing the cast aqueous solution to obtain a frozen mass, thawing the frozen mass and subjecting the thawed mass to one to seven additional cyclic freezing and thawing treatments. The thin membrane can also be prepared by subjecting the frozen mass to partial dehydration in a vacuum until the percentage dehydration rate reaches not less than 3 wt %.