A method for producing a plastic substrate having Tg of 110.degree. C. or more is disclosed, which comprises radical polymerizing a polyfunctional (meth)acrylate compound in a mold by a cast molding method, removing the resulting substrate from the mold when the glass transition temperature of the substrate reaches 10.degree. to 100.degree. C., and then post-curing the substrate. The method of the present invention can provide high quality plastic substrates with high productivity.