To provide a novel geothermal heat exchanging device designing method for determining not only a design with calculation naturally performed by a supplier but also the depth of a pipe to be embedded in the ground and the specification of a pump for circulating heat medium in the pipe in accordance with the result of evaluation test.

The geothermal heat exchanging device uses a heat exchanger for heat exchange between heat in the ground and heat on the ground while using the pump for circulating the heat medium in the heat exchanger and the pipe embedded in the ground. Its designing method for determining the depth of the pipe to be embedded in the ground comprises setting the depth of the pipe to be embedded in the ground to be less than the depth of the pipe which is found by an experiment considering a change in heat efficiency when changing a combination of the specification of the pump and the depth of the pipe to be embedded in the ground where the heat efficiency is started to suddenly lower with respect to the specification of the pump.