The bonding surfaces of an active layer wafer and a supporting wafer have fitting surfaces each comprising a part of a spherical surface of the same curvature, and they are to be bonded together with their bonding surfaces superposed with each other. As a result, an area left as not-bonded in the outer peripheral portion of the bonded wafer is reduced and thus a fixed quality area can be expanded. Therefore, the yield of the bonded SOI wafer becomes high, and the chipping, wafer peel-off and the like phenomenon in the subsequent steps of wafer processing can be reduced.