A combined power generation system improves the generation efficiency of a pressure difference power generation facility by using at least one of air for cooling a turbine of a gas turbine power generation facility and waste heat of flue gas generated by the gas turbine power generation facility. Working fluid to be used in a supercritical fluid power generation facility is cooled by using cold energy of liquefied natural gas. The system includes an air discharge channel via which compressed air is discharged; a fuel gas heater for heating the natural gas to be introduced into the pressure difference power generation facility by performing a heat exchange between the discharged air and the natural gas being heated; and a cooling air inflow channel for guiding the cooled air passed through the fuel gas heater to a turbine of the gas turbine power generation facility.