To provide a heat storage system for storing heat underground at a low cost, and a building.

The heat storage system comprises a fuel cell for generating electric power and the quantity of heat; the building supplied with the electric power and the quantity of heat from the fuel cell; underground piles provided under the building to support the building; heat storage piping provided near the outer periphery inside the underground pile and storing heat in an underground part below the building by allowing the flow of a heat medium heated by the quantity of heat generated by the fuel cell; and an earth retaining part reaching an impermeable layer at least from the lower part of the building while surrounding the underground piles and less permeable than an aquifer. The heat storage piping is located above the impermeable layer.