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.