A method for improving the internal surface of a gas-barrier, when required heat-shrinkable, multi-layer plastics film laminate in the form of a seamless tube having the innermost layer of olefin resin, the method including applying corona discharge employing at least two pairs of electrodes to the innermost layer from the outside of the tube having a gas sealed therein and pressed to a flat state avoiding the contact of opposed surface areas of innermost olefin resin layer with each other, the two pairs of electrodes being arranged such that one electrode of each pair is in contact with the outermost layer of the tube and the other of the same pair out of contact therewith while one electrode of one pair is kept in contact with the outermost layer of the tube with which one electrode of another pair is kept out of contact on the same side of the tube, so that the wetting tension of the innermost layer surface is increased to at least 35 dyne/cm by the corona discharge treatment.