A heat storing system comprises a heat source (1) that generates heat and discharges the heat to a first heating medium and a heat storing unit (2) that has heat storing material (210) to store heat. The heat storing unit has a heat storing material housing section (21) to house the heat storing material and a heating medium housing section (22) to house the liquid phase heating medium. The heat storing material has pores (211) into which heating medium vapor can flow. The heat storing material changes to a solid first phase state when the temperature of the heat storing material is at or below a phase transition temperature and changes to a solid second phase state when the temperature of the heat storing material exceeds the phase transition temperature. The heat storing material stores or discharges heat by the phase change between the first phase state and the second phase state. The heat storing unit is configured to be able to execute a cold discharging mode to discharge cold energy to a cooling medium by way of evaporating the heating medium by exchanging heat between the liquid phase heating medium and the cooling medium in the heating medium housing section.