A reduction treatment using carbon powder for reducing TiO₂ to fabricate non-stoichiometric titanium dioxide, TiO_2−x, was proposed and performed. The carrier density and non-stoichiometric number were calculated by using thermogravimetry (TG) while heating and re-oxidizing TiO_2−x in air. The thermoelectric properties of TiO_2−x were measured and evaluated in air. The results show that TiO_2−x can be simply and safely obtained by reducing insulating TiO₂ through the proposed reduction treatment. The carrier density increases and non-stoichiometric number decrease with the temperature of the reduction treatment and have a good correspondence with the decrease in the electrical resistivity of TiO_2−x. Significantly improved thermoelectric properties were observed as a consequence of the decreasing electrical resistivity of TiO_2−x.