providing a method for manufacturing a semiconductor device , with which it is possible to maintain an isolation breakdown voltage of a locos isolation area high in a semiconductor device finely integrated . a method for manufacturing a semiconductor device , in which a source / drain region is formed by ion implantation on a silicon substrate isolated by a locos isolation area , further comprises a mask forming step to form an implantation mask on the locos isolation area . an implantation mask is formed so as to prevent ions implanted at an ion implantation step from passing through the locos isolation area and arriving at the silicon substrate below the locos isolation area .