Abstract: Object of present invention to overcome a problem of a dispersion of an evaluation of gas barrier property depending on absorbed moisture of plastic, and carry out measurements in which a measurement value response is good and accuracy is good, wherein from the beginning of a container the gas barrier property of a plastic molded body does not depend on the amount of moisture absorbed thereof. A method of measuring the gas barrier property of the plastic molded body such as a plastic container, a plastic sheet or a plastic film or the like which uses a gas analyzer to measure the amount of permeation of a measurement object gas permeating through the plastic molded body, comprising: a heat drying process which heats and dries said plastic molded body in a temperature range which does not cause deformation or heat deterioration.