The thermal relaxation of second-order optical nonlinearity in the subsurface layer of a poled soda-lime silicate glass has been studied. The glass annealing below glass transition temperature was shown to lead to full relaxation of the nonlinearity. At the same time, the measurements of thermostimulated depolarization current demonstrated that spatial electric charge formed in the course of the glass polarization relaxed above the glass transition temperature. This allowed concluding that the second-order optical nonlinearity in the poled glasses was not induced by the spatial electric charge.