A method for manufacturing a bonded wafer by forming an ion implanted layer in a bond wafer; bonding an ion implanted surface of the bond wafer to a surface of a base wafer directly or through a silicon oxide film; and performing a delamination heat treatment. After the formation of the ion implanted layer and before the bonding, a plasma treatment is carried out with respect to a bonding surface of at least one of the bond wafer and the base wafer. The delamination heat treatment is carried out at a fixed temperature by directly putting the bonded wafer into a heat-treating furnace whose furnace temperature is set to the fixed temperature less than 475° C. without a temperature increasing step.