To suppress sectional deformation of an FRP member even if the FRP member has a closed section.

Since a spacer 40 contacting with an inner surface 22 of the FRP member 20 and a fastening collar 30 is disposed within the closed section of the FRP member 20, the spacer 40 can be made to function as a bulkhead (reinforcing material). Therefore, even if a force applied to a fastened member 50 orthogonal to a bolt axis is transferred from the bolt 60 to the fastening collar 30, and then from the fastening collar 30 to the FRP member 20 (even if the FRP member 20 receives an input), the sectional collapse of the FRP member 20 can be suppressed, compared with a case in which no spacer 40 is provided.