This research aimed to evaluate the power generation efficiency of a binary-cycle power-generation system that enables conversion to electric power of hot spring thermal energy in the low-temperature domain (140 degrees C or less), one form of renewable geothermal energy. Power generation output, thermal efficiency, exergy efficiency and exergy destruction were estimated for five working fluids by process calculation using HYSYS. When the basic conditions were as hot spring heat temperature of 60-140 degrees C, flow rate of the hot spring of 200 L/min, heat exchange temperature in the evaporator of 20 degrees C, temperature difference at pinch point of 5 degrees C, and i-butane as the working fluids, a difference in power generation output according to the working fluids was found at hot spring heat temperatures of 90 degrees C and above. In order to obtain 19 kW of power generation output, the exergy destruction of the power generation system was 27 kW, and the exergy destruction of the evaporator accounted for 44% of the whole. When groundwater or river water was used in the cooling process of the condenser, the power generation output was decreased by about 20% with the rise in circulating water temperature in summer.