PURPOSE: To produce barrier layers having a strong barrier effect on adimixtures of polar and non-polar materials by adopting a specified condition at the time of producing the said barrier layers for the said admixtures by exposing them to a fluorine-containing treatment gas on the inner surface of hollow molded parts made of thermoplastics by expanding it in a supporting mold.

CONSTITUTION: At the time of producing barrier layers for admixtures of polar and non-polar materials (for sample: gasoline) by exposing them to a fluorine- containing treatment gas for a few hours on the inner surface of hollow molded parts 9 made of thermoplastics by expanding it in a supporting mold 1, the temperature of the said surface is set to 60 to 250°C at the beginning of the said exposure and the treatment gas 4 is introduced into the hollow molded parts 9 after the gas is cooled preferably to 0 to 196°C by exchanging heat with liquid nitrogen 7 in a heat exchanger 6 to discharge the reaction energy released at the time of fluorination by cooling the treatment gas 4.