An SOI device includes an SOI substrate having a structure in which a first buried oxide layer and a silicon layer are stacked in turn over a semiconductor substrate. A gate is formed over the silicon layer of the SOI substrate. A second buried oxide layer is formed at both sides of the gate in a lower portion of the silicon layer so that a lower end portion of the second buried oxide layer is in contact with the first buried oxide layer. A junction region is then formed in the portion of the silicon layer above the second buried oxide layer so that the lower end portion of the junction region is in contact with the second buried oxide layer.