A semiconductor device having a substrate; an emitter electrode or source electrode formed on the top surface side of the substrate; a gate electrode formed on the top surface side of the substrate; and a collector electrode or drain electrode formed on the bottom surface side of the substrate. The device includes an insulating region formed so as to surround a device-forming region provided on the top surface side of the substrate; and a drift region of the device-forming region, the drift region being in contact with the insulating region, is formed of a semiconductor layer having the same conduction type as that of a channel formed through application of an electric potential to the gate electrode. The gate electrode is a trench gate. An outer peripheral portion of the emitter electrode or source electrode extends in a width of 20 μm or more over the top surface of the insulating region. The insulating region includes, in its interior, a dielectric region having a relative dielectric constant lower than that of the insulating region.