A semiconductor device comprising: a p-type semiconductor substrate; an n-type buried layer formed on the semiconductor substrate; an n-type semiconductor layer formed on the buried layer; a floating potential region provided in a part of the semiconductor layer; a p-type first separation region surrounding the part of the semiconductor layer where the floating potential region is provided, the first separation region being in contact with the semiconductor substrate, and spaced apart from the floating potential region to be formed in a ring-like shape; a diode (Db); and; a p-type second separation region spaced apart from the first separation region to surround, in a ring-like shape, a region where the diode (Db) is located; characterized in that the p-type second separation region reaches beneath the semiconductor layer from a surface of the semiconductor layer; and in that the semiconductor device further comprises: a first insulating separation region provided beneath the semiconductor layer between the floating potential region and the first separation region, wherein the diode (Db) is formed above the first insulating separation region; and a transistor, wherein the transistor is a high voltage n-channel MOSFET, the high voltage n-channel MOSFET comprising: an n-type source region formed in an upper part of the first separation region; an n-type drain contact region formed in an upper part of the semiconductor layer between a cathode region of the diode (Db) and the floating potential region; and a drain drift region including the n-type buried layer beneath the first insulating separation region. a p-type semiconductor substrate; an n-type buried layer formed on the semiconductor substrate; an n-type semiconductor layer formed on the buried layer; a floating potential region provided in a part of the semiconductor layer; a p-type first separation region surrounding the part of the semiconductor layer where the floating potential region is provided, the first separation region being in contact with the semiconductor substrate, and spaced apart from the floating potential region to be formed in a ring-like shape; a diode (Db); and; a p-type second separation region spaced apart from the first separation region to surround, in a ring-like shape, a region where the diode (Db) is located; characterized in that the p-type second separation region reaches beneath the semiconductor layer from a surface of the semiconductor layer; and in that the semiconductor device further comprises: a first insulating separation region provided beneath the semiconductor layer between the floating potential region and the first separation region, wherein the diode (Db) is formed above the first insulating separation region; and a transistor, wherein the transistor is a high voltage n-channel MOSFET, the high voltage n-channel MOSFET comprising: an n-type source region formed in an upper part of the first separation region; an n-type drain contact region formed in an upper part of the semiconductor layer between a cathode region of the diode (Db) and the floating potential region; and a drain drift region including the n-type buried layer beneath the first insulating separation region. a first insulating separation region provided beneath the semiconductor layer between the floating potential region and the first separation region, wherein the diode (Db) is formed above the first insulating separation region; and a transistor, wherein the transistor is a high voltage n-channel MOSFET, the high voltage n-channel MOSFET comprising: an n-type source region formed in an upper part of the first separation region; an n-type drain contact region formed in an upper part of the semiconductor layer between a cathode region of the diode (Db) and the floating potential region; and a drain drift region including the n-type buried layer beneath the first insulating separation region. an n-type source region formed in an upper part of the first separation region; an n-type drain contact region formed in an upper part of the semiconductor layer between a cathode region of the diode (Db) and the floating potential region; and a drain drift region including the n-type buried layer beneath the first insulating separation region.