To provide an inexpensive and highly energy-saving system using underground heat which exchanges heat with outside air by using underground heat at constant temperature throughout the year.

Outside air taken in from an air inlet 1 of a heat exchanger installed by an excavator exchanges heat with underground heat by using water 8 as a heat exchange medium while made to pass through a heat exchange pipe (inflow) 2 and a heat exchange pipe (outflow) 3. The outside air is cooled in summer and superheated in the winter, and is supplied from an air outlet 9 to a supply destination.