We investigated the liquid-liquid phase separation behavior of polyearbonate (PC)/carbon dioxide (CO2) systems by high fidelity digital microscope and light scattering apparatus equipped with a high-pressure visualized cell. When the PC/CO2 system was annealed at a temperature above 200 degreesC, the spherical domain grew and the growth rate decreased with time, suggesting that the liquid-liquid phase separation via nucleation-growth was governed by a diffusion-controlled process under a concentration gradient. In contrast, when the system was annealed at a temperature below 120 degreesC, a co-continuous two-phase structure with a unique periodicity was detected, and the contrast of the structure became greater with time, suggesting liquid-liquid phase separation via spinodal decomposition. The spinodal decomposition was confirmed by analyzing the time evolution of the V-V light scattering intensity based on the linear theory of Cahn. A spherical porous structure was obtained by quenching and depressurization after the liquid-liquid phase separation via nucleation-growth, while an interconnected, continuously extending porous structure was obtained after the spinodal decomposition.