In manufacturing a semiconductor substrate having a dielectric isolation structure, a dielectric film is formed at a semiconductor layer formed by epitaxial growth. Grooves for carrying out dielectric isolation to deposit filler thereon thereafter are used to polish the deposited filler. The polishing condition is obeyed where polishing rate ratio of the filler to the dielectric film is one fifth or less. Thus, an active semiconductor layer in which where elements are to be formed can be provided with good productivity, state where the flatness thereof is good and the layer thickness is uniformly and precisely controlled.