PURPOSE: To obtain a mouth part possessing a screwlike connecting part whose outside is a little longer than the inside and circumferential direction is provided with an undercut, by a method wherein a connecting part of a screw groove of a split mold is formed on a parting face, on which a slope where the outside of a radial direction of the screw groove is made longer than that of the inside of the same is formed.

CONSTITUTION: A mold 1, for example, is made a blow molding mold for a bottle and a connecting part forming part 6 of a screw groove 5a which becomes the lowermost side of a split mold 2 is formed on a parting face 3 of the other split mold 2. The connecting part forming part 6, for example, is formed into a hole-like state. The connecting part forming part 6 is provided with a slope 7. With this construction, the outside becomes a little longer than the inside and an undercut 8 is provided, in a screw thread to be formed with the mold 1. When air is blown into the mold 1 through an air blowing-in part 11, a mouth part 13 of a bottle possessing the screw thread 12 is formed of synthetic resin. Then the undercut 8 of a circumferential direction is formed on a terminal part 14 of the screw thread 12 becoming the lowermost side. Hereupon, as the connecting part forming part 6 is formed on the parting face of the split mold, demolding can be performed smoothly.