To execute a foundation heat-insulating structure having a high termite-preventive function and strength by simple works.

A concrete foundation 12 in which a heat-insulating material 22 in which a fiber body 24, to which termite-preventive chemicals can be absorbed, is stuck on an external surface is arranged on the side of an upstanding section 12b is executed, and the fiber body 24 is impregnated with termite- preventive chemicals and the chemicals are cured. A structure in which the fiber body 24, to which the termite-preventive chemicals can be absorbed, is stuck previously on the external surface of the heat-insulating material 22 is provided as a building material for foundation heat insulation proper to the execution.