A fabrication method of a semiconductor device that can realize a semiconductor device having an improved radiation performance of heat together with a low parasitic capacitance between a semiconductor substrate and a conductor of the device. An SOI structure having a single-crystal silicon layer formed on an insulating substructure is prepared and then, device regions are formed on the substructure by using the single-crystal silicon layer. Sidewall insulators are formed to cover side faces of the respective device regions, laterally isolating the device regions from each other. A resistive silicon layer is formed on a non-device region of the substructure. The resistive silicon layer has a resistivity or specific resistance greater than that of the device regions. Electronic elements are formed in the device regions. The resistive silicon layer may be made of polysilicon or single-crystal silicon.