PURPOSE: To obtain a thermal energy storage material having a desirable temp. range by bringing a bath consisting of a prescribed flame retarding agent and a phase change material and a cementitious matrix material into sufficient contact, thereby forming a composite contg. a prescribed ratio of the phase change material.

CONSTITUTION: The flame retarding agent is selected from a halogen donor compd. soluble in the phase change material (for example, crystalline alkyl hydrocarbon, crystalline fatty acid) and phosphate halide compd. The bath consisting of ≥5% such flame retarding agent and the phase change material is formed. Next, this bath and the cementitious matrix material are brought into contact with each other by spending a sufficient time. The phase change material is then incorporated at 3 wt.% (on the bases of the formed composite) into the composite.