A bottom of a gate trench has a first bottom relatively far from an STI and a second bottom relatively near from the STI. A portion, in an active region, configuring the second bottom of the gate trench configures a side-wall channel region, and has a thin-film SOI structure sandwiched between the gate electrode and the STI. On the other hand, a portion configuring the first bottom of the gate trench functions as a sub-channel region. A curvature radius of the second bottom is larger than a curvature radius of the first bottom. In an approximate center in a length direction of the gate trench, a bottom of a trench is approximately flat, and on the other hand, in ends of the length direction, a nearly whole bottom of the trench is curved.