The polycondensation of aspartic acid in the presence of phthalic anhydride was carried out in mesitylene/sulfolane using o-phosphoric acid as a catalyst. The polymer yields were 91-78%, when 5-20 mol-% phthalic anhydride was added into the feed. The obtained poly(succinimide) carried a phthalic imide unit and a succinic acid unit as end groups. In the MALDI-TOF mass spectrum, the peak-to-peak distances between adjacent signals were 97.07 m/z, corresponding to the calculated value (97.07) of the succinimide unit. Poly(succinimide) was reacted with 2-(methacryloxy)ethyl isocyanate to give end-methacrylated poly(succinimide), in which the end-functionality of the methacrylate group was ca. 1. End-methacrylated poly(succinimide) was polymerized with ethylene glycol dimethacrylate using 2,2'-azoisobutyronitrile to give poly(succinimide) gel, which could be converted into water-absorbing poly(aspartic acid) hydrogel.