PURPOSE: To rupture bridging parts excepting a non-rupture bridging part assuredly when a lid is opened, and rupture weakened lines in the axial direction assuredly also by a method wherein among bridging parts on a weakened line in the peripheral direction, one bridging part which is located on the opposite side in a approximately rectangular direction from a non-rupture bridging part is made into a reinforced bridging part.

CONSTITUTION: A weakened line in the peripheral direction 14 is formed on a skirt wall 6 of a lid main body 2, and it consists of a plurality of slits 26, which extend in the peripheral direction, and a plurality of bridging parts 28a through 28h, which are located between slits 26. One of the bridging parts is made to be a non-rupture bridging part 28a of which the cross sectional area Sa is sufficiently large, and on a pilfer-proof skirt part 18, weakened lines in the axial direction 30a and 30b are formed to be adjacent to both side edges of the non-rupture bridging part 28a. Also, the cross sectional area of a bridging part 28e, which is located at the directly opposite side of the non-rupture bridging part 28a is smaller than Sa but larger than the cross sectional areas of other bridging parts. When the container mouth part is opened, normal bridging parts are ruptured first, then one of the weakened lines in the axial direction is ruptured, and finally, the reinforced bridging part is ruptured.