An apparatus for producing a thin film of tantalum oxide comprising a vacuum chamber with a heater, an ampule of an organic tantalum compound, a container of a carrier gas, a container of an oxygen-containing gas, and a three way valve having a liquid inlet, a gas inlet and an outlet, the liquid inlet being connected with the ampule via a liquid flow controller, the gas inlet being connected with the containers of gas, the outlet being connected with the vacuum chamber, whereby the tantalum compound vaporized at the three way valve can obtain constant supply by the flow controller regardless of the ambient temperature, resulting in the tantalum oxide film of uniform quality.