Heat storage material

A heat storage material comprises NaCH3COO.3H2O as a main component and a nucleating agent comprising 95 to 10 wt. % of anhydrous sodium acetate and 5 to 90 wt. % of at least one of the other sodium salts selected from the group consisting of Na3PO4, Na2HPO4, NaH2PO4, NaNH4PO4.4H2O, Na5P3O10, C6H5Na2PO4.2H2O, NaBO2.2H2O, Na2B4O7.10H2O, Na2C2O4, NaBr, NaCl, CH2(COONa)2.H2O, (CHCOONa)2.H2O, (CH2COONa)2.6H2O, HOC(CH2)2(COONa)2.2H2O, CH3(CH2)2COONa, CH3(CH2)6COONa, HOOCCH(NH2)(CH2)2COONa.H2O sodium oleate, and sodium stearate.