A high withstand voltage lateral semiconductor device capable of improving its on-state breakdown voltage and safe operation area (SOA) without lowering its current capabilities, and structured so as to be easy to produce. The lateral semiconductor device comprises a second conductivity type second semiconductor region formed in a semiconductor layer so as to be adjacent to or away from a first semiconductor region, a second conductivity type source region, a second conductivity type drain region, and a gate electrode formed on a gate insulating film formed between an end of the source region on the surface of the semiconductor layer and an end of the second semiconductor region, wherein the first semiconductor region is extended from under the source region to partly under the gate electrode, the concentration distribution of a first conductivity type impurity increases in the region ranging from the surface of the semiconductor layer to the embedded insulating film and has a peak under the source region, and the impurity concentration in the semiconductor layer ranging from directly under the first semiconductor region to the embedded insulating film is lower than the surface concentration in the first semiconductor region.