To provide a thermal storage tank having a sufficient thermal insulation effect and a heat insulation effect even when the tank is installed at a place having a large change of an external temperature.

The thermal storage tank 1 containing water as a thermal storage medium comprises side faces 12a, an upper surface 12b and a bottom surface 13. A heat insulator 14 is provided at an outside of the tank 1, and a material having a heat insulation such as, for example, a wooden side face partition 15 is provided at an inside on the overall periphery of the side faces 12a while assuring gaps 17a to the side faces 12a. A wooden bottom partition 16 is also provided at the bottom of the tank 1 while assuring a gap 17b to the bottom surface 13. Chilled water or warm water is stored in a storage tank 27 formed of the partition 15 and the partition 16.