To meet a demand for a ground heat exchanger for effectively using huge heat capacity of soil, and a demand for an air conditioner of excellent thermal efficiency operated at an extremely low cost using the ground heat exchanger.

The ground heat exchanger is composed of a plurality of heat exchanger tubes, a supply tank for supplying a heat medium to the plurality of heat exchanger tubes, and a collecting tank for collecting the heat medium from the plurality of heat exchanger tubes. The ground heat exchanger is buried in the ground where temperature hardly changes all the year round or embedded in a heat storage material thermally conductive underground, with a heat insulating material provided between itself and the atmosphere. Air is used as the heat medium, and when outside air is lower than an underground temperature or an indoor temperature is higher than an air-conditioning set temperature, outside air is led into a building via the ground heat exchanger. Cold outside air is thereby warmed close to the underground temperature in winter, while hot outside air is cooled close to the underground temperature in summer to take the outside air into the building. Heating is thereby assisted in winter, and air-conditioning is performed in summer.