A method of manufacturing a semiconductor device. In the method, a substrate is prepared, which includes a buried oxide film and a SiGe layer formed on the buried oxide film. Then, heat treatment is performed on the substrate at a temperature equal to or lower than a first temperature, to form a protective oxide film on a surface of the SiGe layer. Next, the substrate having the protective oxide film is heated in a non-oxidizing atmosphere to a second temperature higher than the first temperature. Further, heat treatment is performed on the substrate thus heated, in an oxidizing atmosphere at a temperature equal to or higher than the second temperature, to form oxide the SiGe layer, make the SiGe layer thinner and increasing Ge concentration in the SiGe layer, thus forming a SiGe layer having the increased Ge concentration.