To easily obtain a depressurized gas space as a heat insulating layer without requiring vacuum facilities.

A composite metal member 5 consists of a plurality of superimposed metal plates 1, 2 which are sealed and joined to each other at a plurality of their peripheries. A coating film 10 is applied to the composite metal member 5 and the whole metal member 5 is heated up to a high temperature during coating. Air between the plurality of closed metal plates 1, 2 is thermally expanded by the heat to give plastic deformation to the metal plate 2. Then, the whole metal member 5 is cooled down to form a plurality of depressurized gas spaces 9 between the metal plates 1, 2 as the metal plate 2 remains plastically deformed.