PURPOSE: To prevent the blocking of a blowing passage of compressed air, which tends to occur at the contraction of the diameter of a parison by pressing, by a method wherein a molding die is closed off after the open end of the parison to be formed is blocked so as to leave the atmosphere in the interior of the parison.

CONSTITUTION: Firstly, thermoplastic synthetic resin melted under heat such as polyethylene, polystyrene or the like is extruded through an extruder die 12, at the center of which a compressed air pipe 11 is equipped, into the form of a tubular parison 13 in an extruder. Secondly, when the length of the parison 13 reaches the predetermined value, molds 1 and 1 are partially closed so as to block the open tip part 13f of the parison 13 by blocking members 3 and 3. At this time, the atmosphere in the parison as it is closed is left in the interior 13a of the parison. Thirdly, when the molds 1 and 1 are closed further, the internal pressure (p) is increased by the atmosphere left in the parison in proportion to the amount of closing of the molds 1 and 1 and consequently a blowing passage 13a with much larger size is formed in a branch part 13b. Accordingly, next when compressed air is blown from a blowing nozzle 11a into the interior 13a of the parison, the clogging of a throat, which tends to develop at a branch part 14a same as 13a, and the ununiform wall thickness or irregular shape, which tends to develop in a container main body 14c inner surface of 13c, can completely be prevented from occurring.