To provide a high-withstand-voltage lateral semiconductor device in which ON-resistance or drain current density is uniform at an end portion and a center portion of the device in a gate width direction. A lateral N-type MOS transistor 11 formed on an SOI substrate includes a trench isolation structure 10b filled with an insulating film at an end portion of the transistor. An anode region 6 of a diode 12 is provided adjacent to a P-type body region 1 of the transistor through the trench isolation structure 10b and a cathode region 15 of the diode 12 is also provided adjacent to an N-type drain-drift region 4 of the transistor through the trench isolation structure 10b so as to cause electric field to be applied to the trench isolation structure 10b to be zero when a voltage is applied across the transistor.