The present invention relates to a fastening tool. The fastening tool of the present invention includes: a male screw-side member having a screw portion the screw portion being formed by alternately arranging, in an outer peripheral direction, male screw surfaces where male screw-side screw threads are formed and outer peripheral pieces where the male screw-side screw threads are not formed; and a female screw-side member having a screw hole, the screw hole being formed by alternately arranging, in an inner peripheral direction, female screw surfaces where female screw-side screw threads are formed and inner peripheral pieces where the female screw-side screw threads are not formed, the screw hole allowing the male screw surfaces and the female screw surfaces to be threadedly engaged with each other.