A water-absorptive soft contact lens material having a high transparency, an excellent flexibility, a high oxygen permeability independent of water content, a high mechanical strength and an adequate hardness, which comprises a copolymer prepared by the polymerization of a monomer mixture comprising an N,N-dialkyl acrylamide or methacrylamide, an N-vinyllactam and a bis(silicon-containing alkyl) fumarate in a specific ratio, and which provides water-absorptive soft contact lenses having excellent properties such as high transparency and high oxygen permeability.