Time evolution of phase separation under an AC electric field was studied for poly(2-chlorostyrene)/poly(vinyl methyl ether) blends containing 0.5 wt % lithium perchlorate (LiClO4) near the critical point. We observed anisotropic morphologies with the interfaces aligned parallel to the direction of the field between two electrodes. The degree of anisotropy (D) was quantitatively evaluated by image analysis of the resulting morphology observed with an optical-phase contrast microscope. We found that D increased at the beginning of the phase separation and tended to reach an equilibrium state, while the characteristic length scale xi of the morphology increased monotonically with time according to a power law xi - t(alpha).