In this study, a mathematical model was developed to simulate the dissolution behavior of a physical blowing agent in molten polymer in the polymer-starvation zone (polymer-partially-filled zone) of the low-pressure physical foam injection molding process (SOFIT^®). The mathematical model was derived by dividing the processing time into two parts : the screw rotating period (metering process), and the waiting period (other processes). In the metering process, the physical blowing agent is assumed to diffuse and flow from the polymer surface to the polymer core in the polymer starvation zone, and the behavior was modeled by diffusion and advection based on the penetration theory. Since the polymer flow is stopped during the waiting period, only diffusion from the polymer surface to the polymer core was modeled. Under the condition that the length of the polymer-starvation zone and the waiting period were kept constant while the gas delivery pressure and the screw rotation speed were changed, the relative error between the calculated and experimental values of the physical blowing agent concentration was about 10%. Furthermore, the change in the concentration against the changes in the specific surface area and the length of the polymer-starvation zone were successfully estimated. The results indicated that the developed model could simulate the change in physical blowing agent concentration during the screw rotating period. In this model, the lengths of the polymer-filled and polymer-starvation zones are not modeled as a function of the operating variables. Therefore, the calculation error of physical blowing agent concentration became prominent when the operating condition, for example, the barrel temperature or cooling time, was changed and the length of the polymer-starvation zone was influenced by the polymer pressure or gas delivery pressure applied to the polymerfilled zone.