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A strain-scanning second-harmonic generation technique is proposed for high-sensitivity measurement of weak strain in film surfaces or interfaces. The basic idea is the sequential application of tensile and compressive strains to a strained film sample. From the strain-dependent second-harmonic generation (SHG) intensity, the type of the strain can be easily judged from whether the SHG is enhanced or weakened, and its magnitude can be precisely calibrated by an externally applied strain that is known. Thus, the built-in strain of a SiO2/Si interface could be determined as tensile with a magnitude of 3.07×10-4.