The bulk polycondensation of L-aspartic acid (ASP) with an acid catalyst under batch and continuous conditions was established as a preparative method for producing poly(succinimide) (PSI). Although sulfuric acid, p-toluenesulfonic acid, and methanesulfonic acid were effective at producing PSI in a high conversion of ASP, o-phosphoric acid was the most suitable catalyst for yielding PST with a high weight-average molecular weight (M-w) in a quantitative conversion; that is, the M-w value was 24,000. For the continuous process using a twin-screw extruder at 3.0kg h(-1) of the ASP feed rate, the conversion was greater than 99%, and the M-w value was 23,000 for the polycondensation with 10 wt % o-phosphoric acid at 260 degrees C. Sodium polyaspartate (PASP-Na) originating from the acid-catalyzed polycondensation exhibited high biodegradability and calcium-ion-chelating ability. The total organic carbon value was 86 similar to 88%, and 100 g of PASP-Na chelated with 5.5 similar to 5.6 g of calcium ion, which was similar to the value for PASP-Na from the acid-catalyzed polycondensation with a mixed solvent