In an insulating cover 50A according to the present invention, a dimension of each of an outer-circumferential-side cylindrical part 50c and an inner-circumferential-side cylindrical part 50b fitting in step parts 40 and 41 in an axial direction of a fastener body 25 is set large with respect to a tip covering part 50a covering a tip surface 26a of a fastening member 26. As a result, on an outer circumferential side and an inner circumferential side of the fastening member 26, a distance from a member 22 is sufficiently ensured by the cylindrical part 50b and 50c. The tip covering part 50a is thinner than the cylindrical part 50B and 50c. Therefore, when the fastener body 25 and the fastening member 26 are fastened together, the amount of deformation of the tip covering part 50a in a thickness direction can be made small.