A diester monomer is represented by the formula (I) ##STR1## wherein R.sub.1, R.sub.2 and R.sub.3 represent a hadrogen atom, an alkoxy, alkenyloxy or hydroxyalkyloxy carbonyl group having 1 to 20 carbon atoms, or a phosphorylcholine derivative ##STR2## where Y denotes --(CH.sub.2)n, --(CH.sub.2 CH.sub.2 O).sub.n CH.sub.2 CH.sub.2 -- ##STR3## n denotes 1 to 8, m is an integer of 0 or 1, R.sub.4, R.sub.5 and R.sub.6 denote an alkyl or aryl group having 1 to 8 carbon atoms, or a hydroxyalkyl group having 2 to 20 carbon atoms; on the proviso that if R.sub.1 is a hydrogen atom, neither R.sub.2 nor R.sub.3 is a hydrogen atom and at least one of R.sub.2 and R.sub.3 denotes the aforesaid phosphorylcholine derivative group, and that if R.sub.2 is a hydrogen atom, neither R.sub.1 nor R.sub.3 is a hydrogen atom and at least one of R.sub.1 and R.sub.3 denotes the aforesaid phosphorycholine derivative group. A polymer and a water-containing contact lens are obtained by polymerizing a starting component material including the above-mentioned diester monomer of the formula (I). A contact lens processing solution includes the polymer and a solvent for dissolving the polymer.