A silicon wafer includes a principal face for forming electronic devices; an end region; and a tapered region which is located between the principal face and the end region, in which the thickness of the silicon wafer is gradually reduced, and which has a slope that makes an angle of greater than zero degree and less than 9.5 degrees or an angle of greater than 19 degrees with the principal face. An SOI wafer prepared by forming a buried oxide layer in a silicon wafer includes a principal face, end region, and tapered region that are substantially the same as those described above. A method for manufacturing an SOI wafer includes the steps of implanting oxygen ions into a silicon wafer; and heat-treating the resulting silicon wafer such that a buried oxide layer is formed in the silicon wafer.