Early stage spinodal decomposition (SD) of polymer solutions (polypropylene/trichlorofluoromethane) induced by pressure jump was investigated with a light scattering instrument designed for polymer solutions under high pressure and high temperature. The changes in the scattered intensity with time at the early stage SD were found to be approximated by Cahn linearized theory or Cahn-Hilliard-Cook theory. The analyses with the theories yielded the characteristic parameters such as the interdiffusion coefficient D-app and the characteristic wavenumber q(m)(0). Unlike other experiments on the polymer solutions, the pressure dependence of the parameters was found to obey the mean field behavior. This may be because the quench depth conditions attained by the pressure jump were much deeper than those attained in other experiments.