We examined a two-step process concept for PLA polymerization with a small amount of catalyst addition to achieve both a high conversion ratio and a small degree of yellowing. First, we developed a calculation program for the chemical reactions. Next, we carried out beaker-scale PLA polymerization experiments to measure the degrees of yellowing. The measured values were compared to amounts of thermal degradation estimated from the calculations simulating the experiments. Then, bench-scale equipment experiments were carried out on the basis of the obtained conditions. The conversion ratios and the degrees of yellowing of the PLA samples obtained from the bench-scale equipment were compared to the numerical simulation results to verify the process concept and conditions. 170°C was good for a first polymerization temperature to avoid a runaway reaction due to heat generation in the ring-opening polymerization. To minimize the degree of yellowing at conversion ratio of 0.9, we chose 5 h as both the first polymerization time at 170°C and the second polymerization time at 190°C. From bench-scale continuous polymerization experiments, we got PLA that had a conversion ratio of 0.92, a weight average molecular weight of 1.96 × 10⁵, and a b-value of 2.42 to verify the concept of the two-step process.