To obtain the subject heat-storing substance which contains a combination of a mixture of phase-changing substance and fine silica particles with carbon black which can heat the phase-changing substance at a specific ratio and can be rapidly heated without deterioration in the heat-storing ability.

This heat-storing substance comprises (A) 85-99.5wt.% of a mixture of a phase-changing substance, for example, a crystalline saturated hydrocarbon melting at 20-100°C, preferably in an amount of 50-80wt.% based on the mixture and microfine silica particles, for example, hydrophobic or hydrophilic silica particles with particle size of 0.005-1.025μ and(B) 0.5-15wt.% of an additive containing a carbon black, for example, conductive carbon black having particle sizes of 25-35nm and a surface area of 245-260m2/g, preferably in addition, (C)glycerol.