A geothermal system including a multitube vertically-sealed underground heat-exchanger includes: a geothermal well formed by vertically excavating a foundation; a heat pump which is arranged in the foundation, and which includes a circulating pump; and a connection tube, an auxiliary facility, and a multitube vertically-sealed underground heat-exchanger which are buried and installed in the geothermal well, and which are connected to the heat pump such that a thermal fluid thermally restored in the geothermal well is supplied to the heat pump through the circulating pump, and the thermal fluid that has undergone heat exchange in the heat pump is recovered back to the geothermal well and thermally restored therein.