A heat storage apparatus comprising: - a container sealed, with a heat storage material encased in the container , the heat storage material including a thermosensitive polymer gel including a thermosensitive polymer and a solvent selected from the group consisting of water, organic solvents, and compounds of water or organic solvents; and - a heat exchanger housed in the container , and configured to cause heat exchange to be performed between the heat storage material and a heating/cooling fluid to heat or cool the heat storage material , and store heating energy or cooling energy in the heat storage material , the heat exchanger being configured to cause heat exchange to be performed between the heat storage material and a heat utilization fluid to receive heat from the heat storage material and to transfer heat from the heat storage material , - wherein the heat storage material makes a reversible hydrophilic-hydrophobic transition at a lower critical solution temperature, and the solvent included in the thermosensitive polymer gel is kept in a liquid state in a process of hydrophilic-hydrophobic transition, - wherein the thermosensitive polymer has a crosslinked structure, and has, at a molecular terminal of the thermosensitive polymer , at least one functional group selected from the group consisting of a hydroxy group, a sulfonate group, an oxysulfonate group, a phosphate group, and an oxyphosphate group, and - wherein a molar ratio between repeating units that form the thermosensitive polymer , the functional group, and a unit of the crosslinked structure falls within the range of 99:0.5:0.5 to 70:20:10. - a container sealed, with a heat storage material encased in the container , the heat storage material including a thermosensitive polymer gel including a thermosensitive polymer and a solvent selected from the group consisting of water, organic solvents, and compounds of water or organic solvents; and - a heat exchanger housed in the container , and configured to cause heat exchange to be performed between the heat storage material and a heating/cooling fluid to heat or cool the heat storage material , and store heating energy or cooling energy in the heat storage material , the heat exchanger being configured to cause heat exchange to be performed between the heat storage material and a heat utilization fluid to receive heat from the heat storage material and to transfer heat from the heat storage material , - wherein the heat storage material makes a reversible hydrophilic-hydrophobic transition at a lower critical solution temperature, and the solvent included in the thermosensitive polymer gel is kept in a liquid state in a process of hydrophilic-hydrophobic transition, - wherein the thermosensitive polymer has a crosslinked structure, and has, at a molecular terminal of the thermosensitive polymer , at least one functional group selected from the group consisting of a hydroxy group, a sulfonate group, an oxysulfonate group, a phosphate group, and an oxyphosphate group, and - wherein a molar ratio between repeating units that form the thermosensitive polymer , the functional group, and a unit of the crosslinked structure falls within the range of 99:0.5:0.5 to 70:20:10. <EOS>