A method for manufacturing a bonded wafer including the steps of: implanting at least one gas ion of a hydrogen ion and a rare gas ion into a bond wafer from a surface thereof to form an ion-implanted layer; bonding the ion-implanted surface of the bond wafer to a surface of a base wafer directly or through an oxide film; thereafter delaminating the bond wafer at the ion-implanted layer to prepare the bonded wafer having a silicon thin film formed on the base wafer; and performing a flattening heat treatment on the bonded wafer under an atmosphere containing hydrogen or hydrogen chloride, wherein a dopant gas is added into the atmosphere of the flattening heat treatment to perform the heat treatment, the dopant gas having the same conductivity type as a dopant contained in the silicon thin film.