The present invention relates to an SOI substrate as a mask substrate for the charged particle beam exposure of which a silicon oxidized film has a suitable thickness for the fabrication of a mask, a silicon membrane layer has a suitable thickness as a mask membrane and is a low stress membrane having no defects and excellent uniformity and relates to its forming method. The SOI substrate is an SOI wafer which is obtained by forming an oxidized film on a first silicon wafer, forming a separation layer by hydrogen ion implantation into the first silicon wafer via the oxidized film, bonding the first silicon wafer onto a second silicon wafer, and cleaving the first silicon wafer from the second silicon wafer at the separation layer so that a silicon membrane layer is formed on the second silicon layer via the oxidized film. The method of forming the SOI wafer is characterized by comprising a step of causing epitaxial growth of silicon to form a silicon membrane layer on said silicon membrane layer simultaneously with doping either or both of boron and phosphorus.