A high breakdown voltage semiconductor device includes a semiconductor substrate, an insulating layer formed on the semiconductor substrate, an active layer formed on the insulating layer and made of a high resistance semiconductor of a first conductivity type, a first impurity region of the first conductivity type formed in the active layer, and a second impurity region of a second conductivity type formed in the active layer and spaced apart from the first impurity region by a predetermined distance. The first impurity region is formed of diffusion layers. The diffusion layers are superimposed one upon another and differ in diffusion depth or diffusion window width, or both.