We investigated the sputtering of the tantalum oxide (Ta2O5) thin films to find suitable conditions for the fabrication of the electrochromic all-solid-state switchable mirror glass using only magnetron sputtering. In particular, we examined the mixture gas ratio of argon, oxygen, and hydrogen used in the preparation of the Ta2O5 thin films by reactive dc magnetron sputtering. The fabricated device could switch its optical state between the reflective (mirror) and transparent states as a result of protons contained in the Ta2O5 thin film. The electrochemical properties of the Ta2O5 thin film were evaluated by the conventional ac impedance method, and the proton conductivity of the films was related to the optical switching properties of the device. When used in the sputtering process, mixture gases containing a large amount of hydrogen led to low proton conductivity in the films, and consequently, the devices exhibited slow switching speeds and poor durability in cyclic switching tests. The mixture gas ratio of P-H2/(P-Ar+P-O2+P-H2)=0.05 in reactive dc magnetron sputtering was suitable for fabricating the device.