Abstract: The present invention provides a fastening method for fastening a first member to a second member by a bolt, comprising the steps of: inserting the bolt through a hole in the first member into a hole in the second member to start threadably engaging the bolt in a threaded hole provided in the second member; then, in a subsequent threadably engaging process, shaping, for example by plastically deforming or cutting, the holes of both the first and second members by a large-diameter portion provided around a bolt shank and having a diameter larger than that of the holes. The invention also provides a fastening system and a bolt used for the above-described method. With this method, it is possible to accurately connect the first and second members by the bolt, and to miniaturize the parts without increasing costs or lowering productivity.