PURPOSE: To furnish a mold enabling the preparation of a double-wall molded article of uniform thickness, by dividing the wall members and the base member of a female mold from each other and thereby moving a parison smoothly along the inner walls of peripheral wall mambers without being restricted when blow molding of the parison is performed.

CONSTITUTION: Out of a pair of male and female molds 10 and 11 for blow molding, the female mold 10 is composed of four peripheral wall members 14 divided from one another at the positions of the ridge lines 12 and 13 of each root, and a base member 15. The base part of each peripheral wall member 14 is maintained swingably between a position whereat it is opened outward in the shape of truncated chevron and a position whereat it approaches the base member 15 and the adjacent peripheral wall members 14 and forms the together with them, by an axis positioned outside the ridge line 13 with the base member 15 and parallel to said ridge line 13. One side film of a parison P follows the inward swinging motion of the peripheral wall members 14 and contact with the inner wall surfaces thereof, expanding the contacting surfaces sequentially, and thus the parison is formed uniform in thickness and in a profiling manner matching the contours of the inner surface of the female mold.