To provide a semiconductor device capable of restricting the substrate bias effect of a high-side transistor while enhancing the heat radiation property of a low-side transistor.A high-side NMOS transistor 101 is formed in a region S1 on the surface of a SOI substrate 30. A trench 41 surrounds the high-side NMOS transistor 101. SiO2 (first insulator) embeds the trench 41. A low-side NMOS transistor 102 is formed in a region S2 on the surface of the SOI substrate 30 around the trench 41. The side face Sf connecting the region S2 forming the low-side NMOS transistor 102 therein and the backside of the SOI substrate 30 is exposed.