NOVELTY - A coating liquid containing a polysilazane compound is applied and a coating film is formed on a belt-shaped base material (1). The base material is relatively moved with respect to light source (2) of vacuum UV ray, vacuum UV ray is irradiated to the coating film, and a gas barrier layer is formed to obtain a gas barrier film. The illumination intensity of the vacuum UV ray in the coating film surface is 160 mW/cm2 or less from the start of irradiation before completion. The luminance of vacuum UV light in the coating film surface is 50-160 mW/cm2.    USE - Manufacture of gas barrier film for organic photoelectric conversion elements (all claimed) e.g. solar cell elements. Can also be used for packages of electronic devices, organic electroluminescent elements, and LCD elements, and packaging materials of foodstuffs, industrial materials, and pharmaceuticals.    ADVANTAGE - The method is suitable for manufacturing gas barrier films having excellent gas barrier property by roll-to-roll method, with high productivity.    DETAILED DESCRIPTION - A coating liquid containing a polysilazane compound is applied and a coating film is formed on a belt-shaped base material. Light sources of vacuum UV ray having uniform illumination intensity and opposing the base material are brought in the width direction of the base material, and the base material is relatively moved with respect to the light source. Vacuum UV ray is irradiated to the coating film and a gas barrier layer is formed, to obtain a gas barrier film having a gas barrier layer containing silicon oxide, on a base material. The illumination intensity of the vacuum UV ray in the coating film surface is 160 mW/cm2 or less from the start of irradiation before completion. The luminance of vacuum UV light in the coating film surface is 50-160 mW/cm2. The energy amount (E1) of vacuum UV ray in the coating film surface received during a period (T) is 180-1800 mJ/cm2.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic cross-sectional view of the vacuum UV ray irradiation apparatus.    Base material (1)    Light source (2)    Exterior electrode (3)    Irradiation chamber (4)    Back roll (5)