To provide an indoor air conditioning system using geothermal heat can cool the inside of a room by outside air passing through a geothermal heat exchanging part in summer, for example, and furthermore, cool the inside of the room by directly feeding outside air into the room, and maintain an indoor cooling action by the outside air passing through the geothermal heat exchanging part over a long period of time.

When the outside air enthalpy is higher than each enthalpy of an air outlet 9 of the geothermal heat exchanging part 4 and a room 1, an air conditioning mode using geothermal heat for feeding outside air passing through the geothermal heat exchanging part 4 into the room 1 is formed, and when the outside air enthalpy is lower than each enthalpy of the air outlet 9 of the geothermal heat exchanging part 4 and the room 1, the outside air is directly fed into the room 1, and an air conditioning and underground cold storage mode directly using outside air for feeding outside air directly into the room 1 and into the geothermal heat exchanging part 4 for underground cold storage is formed.

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