To provide a gas-leak protection film having highly uniform thickness, visibility transmittance, oxygen transmittance and chemical composition, in a flow direction of the film, with a high productivity.

The method for manufacturing the gas-leak prevention film comprises the steps of: forming the gas-leak protection film on a continuously traveling flexible substrate with a vacuum film-forming method; continuously measuring the film thickness by irradiating the gas-leak protection film with X-rays emitted from an X-ray generating device, and detecting fluorescent X-rays or reflected X-rays secondarily generated from the gas-leak protection film; continuously measuring the visibility transmittance of the gas-leak protection film by irradiating the film with the visible rays (wavelength λ=380 nm to 780 nm) generated from the visible light-generating device, and detecting the visible light transmitting through the gas-leak protection film; and calculating a difference between measured results and desired values on the measured film thickness and the visibility transmittance, and adjusting a travelling speed of the substrate, which is a manufacturing condition for the gas-leak protection film.