To obtain a high breakdown voltage semiconductor device of a structure, where reduction in the breakdown strength of the device and changes due to aging the breakdown voltage can be prevented.

An n+ source region 2 is selectively formed in the surface layer of a p-type substrate 1, an n+ drain region 5 is selectively formed in the surface layer of an n-type well region 4, a thermal oxide film 8 is formed in the region 4, an n-type polysilicon field plate 9a is formed on the film 8, first and second connection conductors 14 and 15 are respectively formed on both end parts of this plate 9a and a first conducting film 17 is formed on the plate 9a holding an interlayer insulating film 16 between the plate 9a and the film 17. This film 17 is connected with a source electrode 12 and a first connection conductor 10, a drain electrode 13 and a second connection conductor 15 are connected with a second conducting film 18, a passivation film 19 is covered on the film 17, the film 18 and the exposed place of the film 16, and a molding resin 20 is covered on the film 19.