Surface second-harmonic generation (SHG) of Si(III)-7 x 7 shows an increase in intensity for the surface-state transition (56%) and the strain-induced E-0' interband transition (32%) in response to the phase transition to "1 x 1" taking place around 1100 K. The SHG surface-state transition in "1 x 1" is assigned as the redshifted S-3 --> U-1 transition in 7 x 7 from the observation of no discernible changes in the resonant characteristics. From the symmetry and atomic geometry of the electronic states responsible for SHG,the intensity jump is related to the dissolution of the stacking fault in the "1 x 1" phase.