Heat reservoir comprising porous substrate impregnated with latent heat storage material and method for producing the same

An object of the present invention is to prevent efflux of a heat storage material composition from a porous substrate in a heat reservoir. The heat reservoir of the present invention is characterized in that a porous substrate is impregnated with a latent heat storage material and a hydrogenated styrene-based thermoplastic elastomer. When the weight-average molecular weight of the elastomer is designated as X×10 4 and the amount of the elastomer is designated as Y parts by mass relative to 100 parts by mass of the latent heat storage material in the heat storage material composition， the following conditions are satisfied: 5≤X≤17; 5≤Y≤25; if 5≤X<10， Y≥??2X+25; and if 14<X≤17， Y≤??5X+90.