A Co--Cr--Mo-based alloy includes: 63 mass %.ltoreq.Co<68 mass %; 15 mass %.ltoreq.Cr<26 mass %; 10 mass %.ltoreq.Mo<19 mass %; and the balance of inevitable impurities, wherein the total amount of Cr and Mo is from 32 mass % to 37%. The mass magnetic susceptibility of the alloy is 7.times.4.pi.10.sup.-9 m.sup.3/kg or less at room temperature, and the Vickers hardness number (Hv) of the alloy is 400 or more.