PURPOSE: To provide the titled polyolefinic or polystyrenic, heat insulating body which is formed by adding a material having a specific radiation absorption rate to prevent the increase in its heat conductivity as the expansion ratio increases.

CONSTITUTION: A material having >0.3 mean radiation absorption rate at 10μ thickness basing on blackbody radiation at 300°K (about 27°C or near room temperature), such as polyvinyl alcohol or calcium phosphate is used. Such a material is used in amount of 1W200pts.wt. (preferably 1W50pts.wt.) to be added to 100pts.wt. of a polyolefinic or polystyrenic polymer which absorbs infrared ray of 5W30μ wavelength. Then, the mixture is blown up. Preferably, the expansion ratio is set above the value at which the expanded body would show the minimum apparent conductivity without adding the above described material.