To improve fastening workability by improving the fastening structure of a closed cross section member.

In the fastening structure of a closed cross section member 1, a bolt penetrating hole 2 is formed in a face 11, which is one of the faces of the closed cross section 1, which oppose to each other, and a service hole 3 is formed at a position corresponding to the bolt penetrating hole 2 in the other face 12, and a bolt 5 is inserted into the bolt penetrating hole 2 from the side of the service hole 3, and one of the face 11 of the closed cross section member 1 is fastened by the bolt 5 to a member 6 to be mounted. Projections 4 projected toward the inside of the closed cross section are formed in whole of the periphery of the peripheral edge of the opening of the service hole 3 so that the bolt 5 is prevented from falling in the closed cross section member 1 by the projections 4 during bolt fastening work.