A cap nut to be enveloped in a casting made of casting material includes a substantially cylindrical body. The body has a first end and a second end adjacent to a top end surface of the body. The body has a hole and an outer peripheral surface to be enveloped in the casting material. The body has an exposed surface adjacent to the hole at the first end thereof, and the exposed surface is exposed to an outside of the casting. The body has formed in the outer peripheral surface thereof a tapered surface that broadens from the second end of the body toward the first end. The body has formed in the tapered surface a retaining means for preventing the cap nut from falling off from the casting. The body is installable in and removable from a pattern for forming a sand mold at the tapered surface.