To obtain a heat storage material to be packed to a thermal stratification type heat storage tank enabling a large difference in density between inflow and outflow water even if a large difference in temperature is not made hardly causing a disorder of thermal stratification even in occurrence of difference in flow velocity between feeding water and discharging water or in addition of vibration.

The heat storage technology is attained by packing a dispersion of a heat storage material of latent heat to the thermal stratification type heat storage tank, especially by packing a dispersion of fine particles of a heat storage material having 5-500 mPa.s viscosity at 25°C. Preferably the fine particles of the heat storage material are microcapsules containing the heat storage material, the dispersion has pH 4-11 and the heat storage material is an aliphatic hydrocarbon compound.