The electronic states of oxygen vacancies (VO s) in amorphous oxide semiconductors are shallow donors, deep donors or electron traps; these are determined by the local atomic structure. Because the amorphous phase is metastable compared with the crystalline phase, the degree of structural disorder is likely to decrease, which is referred to as structural relaxation (SR). Thus SR can affect the VO electronic state by changing the local atomic conditions. In this study, we demonstrated that electron doping is possible through the SR of amorphous oxides without redox reactions using a novel device structure that prevents extrinsic reactions with electrodes and ambient atmosphere during annealing. The concentration of VO s in the shallow-donor state in amorphous In-Ga-Zn-O (a-IGZO) increases from ~1016 to ~1019 cm−3 with increasing annealing temperatures between 300 and 450 °C. The SR-driven doping effect is strongly dependent on the annealing temperature but not on the annealing time. The Arrhenius activation energy of the SR-driven doping effect is 1.76 eV, which is similar to the bonding energies in a-IGZO. Our findings suggest that the free volume in a-IGZO decreases during SR, and the VO s in either deep-donor or electron-trap states are consequently transformed into shallow-donor states.