To provide an energy-saving constant-temperature ventilation system effectively utilizing underground heat with a simple constitution.

This energy-saving constant-temperature ventilation system is composed of a heat exchanging mechanism, an air sucking means and an air discharging means, the heat exchanging mechanism is composed of an outside air heat exchanger, and an antifreeze liquid circulation pump connected with the outside air heat exchanger at its both end portions, and provided with a circulation pump at a proper part for circulating the antifreeze liquid. A part of the antifreeze liquid circulation pipe is inserted into a vertical cave buried in the ground to allow the antifreeze liquid to exchange heat with underground heat, and the air exchanging heat by the outside air heat exchanger is supplied from a supply opening of a living room portion by the air sucking means. A steel pipe pile driven in foundation improvement is preferably utilized as the vertical cave formed in the ground, however, a vertical cave formed independently from the foundation improvement may be also used as the vertical cave formed in the ground. Further the order of disposing the outside air heat exchanger and the air sucking means is not particularly restricted, however, the air sucking means is preferably disposed in front of the outside air heat exchanger with respect to the airflow.