To provide a hybrid type geothermal heat utilization system capable of surely supplying necessary quantity of heat even in peak, remarkably reducing the installation cost and being stably operated even in unexpected heavy snow.

This system comprises an underground heat exchanger 1 for exchanging the heat between geothermal heat and a fluid heat medium flowing inside, and a heat medium tank 5 connected with the underground heat exchanger for storing the heat medium, and a heating means is mounted in the heat medium tank for heating the heat medium in the heat medium tank by using an external heat source. A temperature sensor 12 for detecting a temperature of the heat medium and a controller 7 for controlling the operation of the heating means are further mounted, and the controller controls the operation of an electrothermal heater on the basis of the temperature detected by the temperature sensor. Further the controller operates the electrothermal heater when the temperature becomes lower than a specific temperature determined on the basis of a freezing temperature of the heat medium or a specific temperature determined on the basis of an evaporation limit temperature of a heat pump. For example, the heating means is composed of a hot water heat exchanger applying the electrothermal heater 6 or the hot water as its heat source.