To facilitate heat insulating execution and maintenance at low cost even in the case where clearances between pipings and with the pipings and wall bodies are small, and in the case where the pipings are complex by filling comparatively small and many particle shaped heat insulating materials into a container capable of housing pipings to sufficiently fill into even small clearances between pipings and between the pipings and the container.

After a connecting work of each part of pipings 1 in a container main body 21 for housing a header 12 connected to a piping 11 disposed on a front of a header and a branch pipe 13 is completed, heat insulating materials 3 formed by mixing large diameter heat insulating materials 31 and small diameter heat insulating materials 32 made of low water-absorbing foaming resin such as independent bubbles are filled into a container main body 21, and then, a container cover 22 is fixed by a container cover fixing screw 23. It is thus possible to provide a heat insulating structure simply and in low costs comparing with a conventional arts. The small diameter heat insulating materials 32 for composing the heat insulating materials 3 are entered into clearances between large diameter heat insulating materials 31 of low costs, and into small clearances between piping 1 and the container main body 21, and thereby, a clearance is lessened comparing with the case of only the large diameter heat insulating materials 31 so as to further improve heat insulating property.