PURPOSE: To prevent the sectional thickness of a corner part and a bottom pinch-off part from being thinned and thereby to make the whole thickness uniformed molded articles, by a method wherein a parison extruded from an extruder is cut off in a prescribed length by upper and lower splits, and then is subjected in a slackened state to blow molding.

CONSTITUTION: A parison is extruded from a crosshead die 1a for blow molding fitted to an extruder 1, and then is cut off by upper cut-off edges 7a and 7b and the lower cut-off edges 9a and 9b to be a parison (a) having a prescribed length l. Next, by operating a vertically-driving mechanism 12, the left and right half parts 4a and 4b of a lower split 4 are raised simultaneously so as to make the lower contact parts 11a and 11b of an upper split 3 and the upper contact parts 10a and 10b of the lower split 4 contact with each other so that the parison (a) is slackened. Then, pressurized air is blown into the parison (a) from an air nozzle fitted to the crosshead die 1a so as to inflate the parison (a). The parison (a) is thereby made to stick closely on a cavity wall formed of the upper and lower splits 3 and 4, and then is cooled to be cured. A bottle-shaped hollow article thus prepared is taken out by opening the mold.