A semiconductor device having a trench gate structure is provided. A semiconductor device is provided, including: a first-conductivity-type drift region provided in a semiconductor substrate; a first-conductivity-type accumulation region provided above the drift region and having a higher doping concentration than the drift region; a second-conductivity-type base region provided above the accumulation region; and an electric-field relaxation layer provided between the accumulation region and the base region and having a lower doping concentration than the accumulation region. The electric-field relaxation layer may include a first-conductivity-type region including a region having a same doping concentration as the drift region.