Minority carrier diffusion lengths are determined fast, accurately, and conveniently by illuminating a surface of the semiconductor wafer (8) with a beam (7) composed of a plurality of light fluxes each having a different wavelength modulated at a different frequency (f1, f2). Surface photovoltages induces by different light fluxes are simultaneously detected by monitoring surface photovoltage signals at the different modulation frequences (f1, f2). The surface photovoltage signals are frequency calibrated and then used to calculated a minority carrier diffusion length.