A semiconductor device is formed on a SOI substrate having a semiconductor substrate, a buried oxide film formed on the semiconductor substrate, and a semiconductor layer formed on the buried oxide film, the semiconductor substrate having a first conductive type, the semiconductor layer having a second conductive type, wherein the buried oxide film has a first opening opened therethrough for communicating the semiconductor substrate with the semiconductor layer, the semiconductor layer is arranged to have a first buried portion buried in the first opening in contact with the semiconductor substrate and a semiconductor layer main portion positioned on the first buried portion and on the buried oxide film, the semiconductor substrate has a connection layer buried in a surface of the semiconductor substrate and electrically connected to the first buried portion in the first opening, the connection layer having the second conductive type, and the semiconductor device includes a contact electrode buried in a second opening, a side surface of the contact electrode being connected to the semiconductor layer main portion, a bottom surface of the contact electrode being connected to the connection layer, the second opening passing through the semiconductor layer main portion and the buried oxide film, and the second opening reaching a surface portion of the connection layer.