To provide an air conditioning system using both underground heat and outdoor air heat based on the level of efficiency in heat exchange and having high COP.

In this hybrid air conditioning system using underground heat, an air heat exchanger is incorporated into a heat pump using underground heat. When the underground temperature is higher than (outdoor air temperature -; is a positive constant) during use for cooling in summer and when the underground temperature is lower than (outdoor air temperature +; is a positive constant) during use for heating in winter, circulation on an underground heat source side is stopped, and operation as an air heat source heat pump is performed.