PURPOSE: To provide a MOS semiconductor device having extermely low floating capacity and high speed operation by forming the bottom surface of source and drain regions in contact with an insulator.

CONSTITUTION: An N-type monocrystalline silicon layer 2 is formed on a p-type monocrystalline silicon substrate 1, and wafer formed with phosphorus glass layer 8 on a silicon substrate 5 and wafer formed with a phosphorus glass layer 8 on the layer 2 are adhered on both sides of the layer 8 in facing manner with each other. Then, porous substrate 1 is provided, and converted to oxide and removed thereform. Then, after a silicon oxide film 10 and a polycrystalline silicon layer 11 are formed, D-type regions 12, 13 are formed thereat by etching. Then, source and drain regions 12 and 13 are formed until reaching the boundary between the layer 2 and the layer 8, and an oxide film 14 is simultaneously formed to thereby form gate electrodes 15∼17.