A novel methacrylic acid ester derivative, methyldi(trimethylsiloxy)sylylpropylglycerol methacrylate which is availably employed as a main monomer for preparing copolymers useful as a material of contact lenses. The derivative has siloxane bond which contributes to increase of the oxygen permeability of its copolymers, and has hydrophilic hydroxyl group and ether bond by which the derivative is miscible with a hydrophilic monomer in all proportions and is easily copolymerizable therewith and colorless transparent copolymers are obtained.