A copper alloy with an excellent stress relaxation resistance including Ni: 0.1 through 3.0 mass%, Sn: 0.01 through 3.0 mass%,P: 0.01 through 0.3 mass% and remainder copper and inevitable impurities, and the Ni content in extracted residues separated and left on a filter having filter mesh size of 0.1 µm by using an extracted residues method accounting for 40 mass% or less of the Ni content in the copper alloy, wherein the extracted residues method requires that 10 g of the copper alloy is immersed in 300 ml of a methanol solution which contains 10 mass% of ammonium acetate, and using the copper alloy as the anode and platinum as the cathode, constant-current electrolysis is performed at the current density of 10 mA/cm 2 , and the solution in which the copper alloy is thus dissolved is subjected to suction filtration using a membrane filter of polycarbonate whose filter mesh size is 0.1 µm, thereby separating and extracting undissolved residues on said filter, and the Ni content in the extracted residues is identified through analysis by ICafter dissolving said undissolved residues separated and left on said filter into a solution prepared by mixing aqua regia and water at the ratio of 1 : 1.