To provide an external heat insulating structure easily constructible on the roof of a building at a low cost.

Granular foam glass materials 5 of predetermined size are stored in a air and water permeable container 8 to constitute a heat insulating unit 6, and the heat insulating unit 6 is laid on the surface of a building skeleton 1 to form a heat insulating layer 7, thus constructing the external heat insulating structure on the skeleton 1. In the case of the skeleton 1 being formed of concrete, a waterproof layer formed of two layers consisting of a primary waterproof layer 2 and a secondary waterproof layer 3 is formed between the skeleton 1 and the heat insulating layer 7.