To provide a fastening method achieving a target resonance frequency of a vehicle body in a fastening operation at a specified torque by interposing the automobile subframe rigid collar.

In the fastening method, after inserting from the thread section side of a fastening bolt 10 through a fastening bolt insertion hole 24 of a cylindrical section 23 that becomes a collar body of the automobile subframe rigid collar 20, the front end side of the thread section of the fastening bolt 10 is set while being screwed into a tapped hole 2 of a monocoque body 1, and the fastening bolt 10 is fastened at the specified torque. A tapered outer peripheral section 21 of the automobile subframe rigid collar 20 is compressed and press fitted into a guiding hole 3 while deforming an outer peripheral section having a larger diameter than the inner diameter of a through-hole 5 by plasticity that is the characteristic of a molding material of the automobile subframe rigid collar 20. Thus, the fastening is carried out by press fitting and press bonding deformed portions 21 and 22 of the tapered outer peripheral section into between the inner peripheral surfaces of the guiding hole 3 and the through-hole 5 and the outer peripheral surface of the fastening bolt 10.