To provide a mounting structure of a keeper for a denture attachment which structurally enhances the attraction force of the denture attachment and is mountable to an implant with high tightening force.

The keeper 10 comprises a keeper body part 13 having an attraction surface 12 and a shaft part 15 disposed on the surface on the side opposite to the attraction surface 12. The keeper body part 13 has a flat surface 14 for engaging a rotary tool on its outer periphery and is provided with the attraction surface 12 on the flat surface free of the ruggedness. The implant 4 has a mounting hole 435 screwable to the shaft part 15. A ring member 2 is interposed between the keeper body part 13 and the implant 4 and the shaft part 15 of the keeper 10 is screwed to the mounting hole 435 of the implant 4, by which the keeper 10 is mounted at the implant 4.