The early stage of spinodal decomposition of partially miscible poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) blend was studied by time-resolved synchrotron radiation small-angle X-ray scattering (SR-SAXS). Intensity change of a scattering peak corresponding to the concentration fluctuation was traced after rapid heating to the two-phase region from the miscible state. At the initial stage, the dominant phase size was kept constant on the order of tens of nanometers, and then, grew to the order of sub-microns during decomposition. The apparent diffusion constant was determined according to the Cahn-Hilliard theory.