1. A method of measuring residual stress of material using an instrumented indentation technique, comprising the steps of: repeatedly applying and removing an indentation load to and from a reference specimen of the material in a stress-free state, thus generating an indentation load-depth curve in a stress-free state; measuring a maximum indentation load and a maximum indentation depth from the indentation load-depth curve in a stress-free state; calculating an actual indentation depth (h c ) from the maximum indentation depth and calculating a contact area (A c ) from the actual indentation depth; and measuring hardness (H IT ) of the material from the contact area, thus generating a stress-free curve (stress reference curve) for the reference specimen; repeatedly applying and removing an indentation load to and from an actual specimen of the material in a stressed state, thus generating a stressed curve for the actual specimen; and measuring the residual stress of the material using both a load difference (ΔL) obtained from comparison between the stress reference curve and the stressed curve and a contact area (A s ) in a stressed state obtained from a loading curve.