We investigated the viscoelastic effects on the early stage spinodal decomposition (SD) in semi-dilute polymer solutions where the coupling between stress and diffusion plays an important role. The so-called viscoelastic length, xi (ve), within which the stress suppresses the growth of the concentration fluctuations, was quantitatively evaluated experimentally. The evaluated value was found out to be about 10 times larger than the radius of gyration of polymer. We measured the interdiffusion coefficient, the cooperative diffusion coefficient, the zero-shear viscosity, and the plateau modulus in the one phase region and evaluated xi (ve) independently from the SD experiment. The value xi (ve) obtained in the early stage SD agrees well with that estimated from the diffusion and viscoelasticity measurements, confirming the validity of the Doi-Onuki theory.