To form an isolation channel of stable quality with high yield, in the manufacture of a semiconductor device which includes a process for forming an isolation groove for dividing the active region on a semiconductor substrate.

On a semiconductor substrate, a silicon nitride film is formed which serves as a polish stop film. In a specified region, the silicon nitride film and the semiconductor substrate are removed by etching to form an isolation groove for dividing an active region. Thereafter, a silicon oxide film is so deposited above the semiconductor substrate as to fill the isolation groove. Next, a first stage CMP is conducted with an abrasive material, including SiO2, with which the surface of the silicon oxide film can be polished with good efficiency, regardless of the step difference in level. Then, a second stage CMP is conducted with an abrasive material, including CeO2 having a large polish selectivity with respect to the silicon oxide film and the silicon nitride film.