After simultaneous applications of ultraviolet irradiation and a sufficiently high electric field, the so-called W-poling, silica glass doped with germanium shows quite a large second-order optical nonlinearities in both bulk and film samples, and thus induced d coefficient is comparable to there of LiNbO3 crystal. The induced effect decays after the W-poling with exactly a single exponential function of time, suggesting a kind of ordered structure formed in the glass by the UV-poling. The decay rate strongly indicates Ge-E ' centers to be relevant agents to the nonlinearity. The decay or degradation can be made much slower with addition of impurities as electron scavengers. Mechanism to give the nonlinearity will be discussed, relating to generation of crystallites in the glass.