A Co—Cr—Mo alloy with nitrogen addition composed of 26 to 35% by weight of Cr, 2 to 8% by weight of Mo, 0.1 to 0.3% by weight of N, and balance of Co is subjected to solution treatment and then subjected to isothermal aging treatment holding the alloy at 670 to 830° C. for a predetermined period of time to form a multi-phase structure composed of an ε-phase and a Cr nitride by means of an isothermal aging effect. After cooling, the alloy subjected to reverse transformation treatment in which the alloy is heated at a temperature range of 870 to 1100° C. for reverse transformation to a single γ-phase from the multi-phase structure composed of an ε-phase and a Cr nitride.