The invention prevents a source-drain breakdown voltage of a DMOS transistor from decreasing due to dielectric breakdown in a portion of a N type drift layer having high concentration formed in an active region near field oxide film corner portions surrounding an gate width end portion. The field oxide film corner portions are disposed on the outside of the gate width end portion so as to be further away from a P type body layer formed in the gate width end portion by forming the active region wider on the outside of the gate width end portion than in a gate width center portion. By this, the N type drift layer having high concentration near the field oxide film corner portions are disposed further away from the P type body layer without increasing the device area.