An object of this invention is to effectively reduce a connection resistance between a plug and an interconnect, and a dielectric constant of an insulating film. A semiconductor device 100 has a semiconductor substrate (not shown); a first interconnect 108 made of a copper-containing metal which is formed over the semiconductor substrate; a conductive first plug 114 formed over the first interconnect 108 and connected to the first interconnect 108; a Cu silicide layer 111 over the first interconnect 108 in an area other than the area where the first plug 114 is formed; a Cu silicide layer 117 over the first plug 114; and a first porous MSQ film 105 formed over an area from the side surface of the first interconnect 108 to the side surface of the first plug 114 and covering the side surface of the first interconnect 108, the upper portion of the first interconnect 108 and the side surface of the first plug 114.