An anti-theft fastening device for fastening an SBW motor externally mounted to a transmission case that includes a bolt having a head portion and a threaded portion integrally formed with the head portion, a bottomed tubular bolt cover having an opening end, and a cap member closing the opening end of the bolt cover. The bolt cover has a through bore to allow the threaded portion of the bolt to pass through, a bottom portion abutting a bottom surface of the head portion of the bolt, and a grip portion projecting from an outer circumference of the bottom portion and surrounding a periphery of the head portion of the bolt. When the threaded portion of the bolt is screwed into a threaded groove of the transmission case, the bottom portion of the bolt cover is interposed between the bottom surface of the head portion and each fastening portion of a bracket portion.