PURPOSE: To provide a structure for mounting a reinforcing sleeve to a steel material which is capable of reducing the cost and enhancing the workability.

CONSTITUTION: A reinforcing sleeve 11 comprises a sleeve main body 15 and a pair of flanges 16 and 17 integrally formed on the sleeve main body 15. Each of the flanges 16 and 17 is caulked and formed around the central part of the sleeve main body 15 in the axial direction so that the sleeve main body may bulge out in the radial direction from an inner peripheral surface in which an insertion hole 14 is made in a state that the sleeve main body 15 is inserted into the insertion hole 14. A peripheral edge of the insertion hole 14 made in a web area 13 of a beam 12 is clamped between each of the flanges 16 and 17, thereby fixing the reinforcing sleeve 11 with the beam 12.