To provide a geothermal apparatus, not requiring formation of a hole for burying a heat exchange pipe to a depth of 20m-60m underground, and hardly causing a sudden water temperature change in a rainwater storage facilities to operate over a long period of time.

This geothermal apparatus includes: an indoor air conditioner 18; rainwater storage facilities 3 formed underground; a heat exchange tank 4 formed underground and provided with rainwater circulating pipes 11, 12 disposed for circulating the rainwater in the rainwater storage facilities 3 in a space up to the rainwater storage facilities 3; a control device 6 for circulating the rainwater in the rainwater storage facilities 3 between the rainwater storage facilities 3 and the heat exchange tank 4 when the water temperature in the heat exchange tank 4 reaches a specified temperature; and a heat pump 2 provided between the air conditioner 18 and the heat exchange tank 4 and including a heat exchange pipe 5 disposed between the air conditioner 18 and the heat exchange tank 4.