A composite steel plate has a core material and steel plates on both sides of the core material and has mounting holes. A metal collar has a flange, a sleeve portion extending from the flange, and a locking portion extending from the sleeve portion and having a thickness thinner than that of the sleeve portion. The present invention provides a mounting structure for mounting the metal collar onto the composite steel plate. The locking portion is enlarged, and a first enlarged diameter portion is formed. Then the sleeve portion is enlarged, and a second enlarged diameter portion is formed. One steel plate is thereby clamped between the first enlarged diameter portion and the second enlarged diameter portion, and the other steel plate is thereby clamped between the second enlarged diameter portion and the flange. Thus, the metal collar is mounted onto the composite steel plate.