Clearance can be secured, the thrust of a bolt provided with a left-hand external screw thread is stabilized and the costs of component members are reduced in attaching a steering member to a body frame. A bolt-and-nut fastening mechanism includes a right-hand bolt 4 provided with a metal-to-metal contact locking mechanism 7 on a threaded part, a left-hand bolt 5 provided with an internal screw thread 8 with which the bolt engages, and a fixing nut 6 capable of engaging with the left-hand screw thread 9 of the left-hand bolt 5 or a member 14 provided with an internal screw thread with which the left-hand screw thread 9 of the left-hand bolt 5 engages. The locking mechanism 7 of the bolt 4 thrusts the left-hand bolt 5 forward so that the gap between the top surface 13 of the head of the left-hand bolt and an inner panel 2 is narrowed, and the head of the left-hand bolt 5 comes into contact with the inner panel 2 to define a clearance between a body frame and the bracket 3 of a steering member. The bolt 4 is screwed into the left-hand bolt 5 and the base surface 11 of the bolt comes into contact with an outer panel 1 to produce a fastening force. The fastening force is transmitted through the left-hand bolt 5 to attach the steering member. A welded nut 6 may be omitted and an internal screw thread 15 with which the left-hand bolt engages may be formed in the bracket 14 of the steering member.