A method for manufacturing a bonded wafer, including at least implanting at least one type of gas ion selected from a hydrogen ion and a rare gas ion from a surface of a bond wafer to form an ion-implanted layer in the wafer, bonding an ion-implanted surface of the bond wafer to a surface of a base wafer directly or through an insulator film, and then delaminating the bond wafer at the ion-implanted layer to fabricate a bonded wafer. A plasma treatment is applied to a bonding surface of one of the bond wafer and the base wafer to grow an oxide film, etching the grown oxide film is carried out, and bonding to the other wafer is performed. The method enables preventing defects by reducing particles on the bonding surface and performing strong bonding when effecting bonding directly or through the insulator film.