A method of manufacturing a semiconductor device includes forming a first and a second isolation insulating film to define a first, a second, a third and a fourth region, forming a first insulating film, implanting a first impurity of a first conductivity type through the first insulating film into the first, the second and the fourth region at a first depth, forming a second insulating film thinner than the first insulating film, implanting a second impurity of a second conductivity type through the second insulating film into the third region at a second depth in the semiconductor substrate, implanting a third impurity of the second conductivity type into the third region at a third depth shallower than the second depth, forming a first transistor of the first conductivity type in the third region, and forming a second transistor of the second conductivity type in the fourth region.