To restrain lowering of closing property due to heating, provide a sealing function, and easily separate a cap from a container by mounting, on an inside cap having a seal ring, an outside cap made of a synthetic resin having softening temperature and rigidity higher than that of the inside cap and having a softening temperature higher than the boiling point of water.

At a bottom end 11 of a main part 10 of an inside cap 9 formed out of a resin such as a polyethylene, a seal ring 13 is integrally provided through a weakened line 12. An outside cap 7 comprising a cap 16 and an outside cylinder 17 which are made of a synthetic resin whose softening temperature and hardness are higher than that of a synthetic resin forming the inside cap 9 and has a softening temperature higher than the boiling point of water, for example, a polypropylene is mounted in an engaging part 8 of the inside cap 9 by locking a locking part 18. Thus, a cap having a higher part than the main body 10 having a high hardness such an extent that its closing property is not damaged by a heating liquid 31 and the part of the seal ring 13 which can be easily torn and removed and has a hardness lower than that of the outside lid 7 formed integrally with the higher part can be obtained.