To increase the volume of inner space at low cost.

A core die 42 is formed of resin material of a lower fusing point than resin material for forming a chamber container 18 by injection moulding or the like. Next, an this core die 42 is disposed in a moulding die 40, the resin material is injected into the moulding die 40 to mould the chamber container 18. After moulding the chamber container 18, it is heated at such a temperature that the core die 42 only is fused by means of oil jet or a constant temperature tank or the like to fuse the core die 42 from the chamber container 18, thereby completes the chamber container 18.