A semiconductor device is provided with an N−-type drift layer, a P+-type diffusion well region provided on a surface part of the N−-type drift layer, a P-type channel well region, an N+-type diffusion well region, a gate insulating film, a gate electrode laminated on the gate insulating film, a drain trench, a field plate provided in the drain trench with a silicon oxide film and an insulating film interposed therebetween and a field plate electrode formed on the field plate. The field plate is tapered toward a base part of the drain trench. A distance between a side wall of the drain trench and a side face of the field plate is increased toward the base part side.