By measuring the temperature dependences of second harmonic intensity from several (2×1 ) reconstructed St (100) samples with different doping materials and doping concentrations at the foundmental wavelength of 1064 um, we have revealed that the reflected second harmonic signals at this fundamental wavelength is induced by electrons in surface states, not by surface depletion electric field. The intensity of reflected SH signals was inversely proportional to the square of the sample temperature.