The characterization of the cellular structure of plastic foams has long been of importance to investigate their physical properties. We have been developing an instrument that measures the orientation of seat materials based on their dielectric anisotropy. It is demonstrated by electromagnetic theory that composite materials with an island-sea structure show a macroscopic anisotropy of dielectric constant because of the shape of islands, even if both the islands and sea are dielectrically isotropic themselves. The orientation based on the dielectric anisotropy caused by the shape of cells was investigated for the polystyrene by using our microwave cavity resonator method. From this data, a better manufacturing condition to minimize the thermal shrinkage was derived. The manufacturing condition was taken as extrusion rate, foaming temperature and die gap. The shape of cells of polystyrene foams was estimated using the derived equations, which express the maximum and minimum dielectric constant of whole composite materials as a function of the eccentricity of their ellipsoidal islands and the volume fraction and the dielectric constant of each material in the islands-sea structure.