To provide a geothermal heat collecting system capable of saving heat medium transporting power.

This geothermal heat collecting system 1 which comprises an underground vessel 15 mounted under a ground level G and storing a first heat medium M1, and a heat exchanging flow channel 10 having an internal flow channel 11r in which the first heat medium M1 flows, an external flow channel 12r formed to receive the internal flow channel 11r to allow the first heat medium M1 to flow in the direction opposite to the flow in the internal flow channel 11r, at an outer side of the internal flow channel 11r, and in which the heat exchanging flow channel 10 is mounted under the ground level G horizontally or in a state of having a downward gradient when observed from the underground vessel 15, heat exchange is performed between the heat of the first heat medium flowing in the external flow channel and the underground heat, and the first heat medium is convected by the change of concentration of the first heat medium, thus the geothermal heat can be collected without using power.