To provide a simple-structured heat pump system of high system COP using geo-heat capable of contributing to prevention of global warming and increasing the heat collection performance to use geo-heat for snow-melting, space heating, hot-water supplying, etc. at high efficiency.

The heat pump cycle system collecting geo-heat from a heat-collection tube buried in the ground and using the heat as a heat source for an artificial facility is equipped with a heat-collection tube 1 comprising a double tube or U-shaped tube vertically buried in the ground (e), a heat pump 3 taking in geo-heat from the heat-collection tube 1 and transferring it to a heat load of the artificial facility through a heat exchange, and a brine circulation path 4 connecting the heat-collection tube 1 and the heat pump 3 with a pump 2 placed therebetween to circulate brine by force, wherein carbon dioxide CO2 as a brine is circulated through the heat-collection tube 1 to let the sensible or latent heat heat-exchange absorb geo-heat (h).