PURPOSE: To improve productivity and characteristic of a heat insulating agent by mixing coal ash containing less than the specific diameter of grobular shape grain at the specific % and potato waste together with the other blending raw material, forming and next drying.

CONSTITUTION: The coal ash A containing ≤ 44 μ diameter of the globular shape grain at ≥ about 60%, dried powder B of the potato waste and coke powder C, etc., are mixed with a mixer 1. Next, after kneading by adding waste in a kneader 2, the kneaded material is extruded and formed with a forming machine 3 and the dry is executed in a drying furnace 4 by hot blast. By this method, the fine particles of the coal ash remarkably reduce the resistance at the time of forming by playing the role as ball bearing. As the kneading is executed after adding water, the formed strength is improved and by using the potato waste, powdering ratio is reduced during drying. Therefore, production of the obtd. heat insulating agent E is simply facilitated, and the productivity is improved together with the characteristics of the heat insulating and oxygen shut-off.