A semiconductor device includes a semiconductor part, a first electrode and control electrodes at the front side of the semiconductor part. The semiconductor part includes first to fourth layers, first and third layers being of a first conductivity type, second and fourth layers being of a second conductivity type. The control electrodes are provided in a plurality of trenches, respectively. The control electrodes include a first control electrode, and a second control electrode next to the first control electrode. The second layer is provided between the first layer and the first electrode. The third and fourth layers are provided between the second layer and the first electrode. The semiconductor part further includes a first region partially provided between the first and second layers. The first region is provided between the first and third layers, the first region including a material having a lower thermal conductivity than the first layer.