A copper alloy for fastening wherein the alloy has a structure of a mixture of α-phase and a β-phase; and wherein the alloy has a composition represented by the general formula: Cubal.ZnaMnb, where bal., a, and b are expressed in % by mass, bal. represents the balance, 34≤a≤40.5, 0.1≤b≤6, and inevitable impurities may be contained; and the composition satisfying the equation (1): b≥(−8a+300)/7, where 34≤a