An implant overdenture system 1 is used to fix an implant overdenture 3 onto alveolar ridge 8 by the support of implants 10. The implant overdenture 3 has artificial teeth 31, a denture base 32, and magnetic assemblies 4. The implants 10 each have an implant body 11 and a keeper 2. A largest circumscribed circle diameter d1 in an outer shape of a surface-to-be-attracted of the keeper 2 is equal to or larger than φ1.8 mm, and a largest diameter d2 of the implant body 11 is equal to or larger than φ1.2 mm and equal to or less than φ3.5 mm, and these largest diameters have a relationship expressed by d1/d2 ≤ 1.5.
