A gas turbine system (1A) includes a gas turbine unit (2) and a cooling fluid generator (5). The gas turbine unit (2) includes a first compressor (21) and a first expansion turbine (23) coupled to each other by a first shaft (22), a combustor (26), and a fuel tank (30). A fuel held in the fuel tank (30) circulates through a fuel circulation passage (31). A working fluid that has a pressure increased by the first compressor (1) is extracted from the gas turbine unit (2). The cooling fluid generator (5) includes a cooler (55) for cooling, with the fuel flowing through the fuel circulation passage (31), the working fluid that has been extracted from the gas turbine unit (2), and a second expansion turbine (53) for expanding the working fluid that has flowed out of the cooler (55).