The CO2 diffusion coefficient in polymethyl methacrylate (PMMA) was measured to investigate the relationship between the saturation time and cell nucleation in batch foaming. Measurements were conducted using a visualized high-pressure cell at various temperatures from 303 to 343 K and CO2 pressures of 6, 12, and 18 MPa. The CO2 diffusion coefficient depended strongly on the temperature but weakly on the pressure. Additionally, the CO2 diffusion was promoted by the glass transition behavior of PMMA but was suppressed by the hydrostatic pressure at high pressures. Based on the sorption isotherms of CO2 in PMMA, microcellular PMMA foams were prepared with saturation times from 1800 to 12600 s. Cell nucleation was initiated when the CO2 concentration, which depended upon the CO2 diffusion and saturation time, was sufficient to minimize the energy barrier. Furthermore, the critical CO2 concentration to initiate cell nucleation decreased with an increase in temperature.