A method between a diffusion and a quenching process including normalizing by step cooling including alternating temperature lowering and temperature keeping treatment for a temperature history that satisfies a predetermined condition; after normalizing, maintaining the temperature of the whole workpiece so that the whole workpiece becomes the predetermined temperature, thereby producing fine crystal grains in the workpiece; then reheating the workpiece to raise its temperature to the second temperature. There is uniformity in temperature between the surface and the inside of a workpiece, and crystal grains are prevented from being coarse.