Raw materials for a copper alloy are melted in a high frequency smelter and cast, and milling, rolling, and annealing are carried out. Then, rolling is again carried out. Thereafter, the materials are heated at a temperature of 900.degree. C. for one minute and are quenched in water, to be solution treated. Subsequently, the materials are heated at a temperature of 500.degree. C. for five hours for aging, and then are cooled at a cooling rate in a range of 10 to 50.degree. C. per hour until the materials are cooled to a temperature of 380.degree. C.