PURPOSE: To facilitate the separation of a cylindrical part of a container lid by a method wherein when a container is opened by relatively rotating the container and container lid made of a synthetic resin, the cylindrical part of the container lid is assuredly separated with a weakened part as the border, and at the bottom side of a neck part of a male thread on the outer periphery of the neck part of the container, protruding parts which are confronted in the diametrical direction are provided.

CONSTITUTION: At a neck part 10 of a container, a plurality of ratchets are formed at the lower side on the neck part than a male thread 12 on the outer peripheral surface. At the same time, a pair of protruding parts 16, 18 being confronted to each other in the diametrical direction are formed. When the neck part 10 is closed by a container lid 30, the container lid 30 is rotated, and the neck part 10 and container lid 30 are engaged by making contact pieces 32... being formed on the internal surface of a pilferproof skirt part 37 which is connected to a main part 35 of the container lid 30 by a bridge (weakened part) 40 get over the contact surface 14A of the ratchet 14 and the protruding part 16, by the screwing between the male thread 12 and female thread 42. When the lid is opened, since a relative movement between the skirt part 37 and container is blocked, the major part 35 and skirt part 37 are separated by cutting the bridge 40.