To save resources, preserve the environment and suppress the cost by heat-treating an Al dross residual ash, screening the heat-treated Al dross residual ash, providing an Al dross residual ash having a larger particle diameter than a specific value and using the resultant Al dross residual ash as a raw material for a lightweight heat insulating refractory.

An Al dross residual ash is heat-treated to thereby remove nitrogen components and chlorine components remaining in the residual ash. The Al dross residual ash is thereby chemically stabilized and detoxicated. The heat-treated residual ash is screened to provide an Al dross residual ash having >74 μm particle diameter. That is, the resultant lightweight heat insulating refractory has a bulk specific gravity and physical properties nearly comparable to those of a lightweight heat insulating refractory using a chamotte which is the existing material, especially properties of thermal conductivity extremely close thereto. Thereby, no problem is caused even when the lightweight heat insulating refractory is used solely or mixed as a substitute for the existing refractory. As a result, the consumption of natural resources can be reduced and the amount of the disposed Al dross residual ash can be reduced.