To provide a gas expander having high durability and capable of reducing corrosion and damage due to operating fluid.

This gas expander is provided with a nose cone 7 fixed within an inflow nozzle and located on the upstream side of a rotor disc 8, piping for supplying a seal medium to the inside of the nose cone 7, a first channel 21 formed between the nose cone 7 and the rotor disc 8 and communicated with the inside of the nose cone 7 and a fluid flowing path 14, a second channel 22 provided at an open tip side inner edge part of the nose cone 7 and communicating with the inside of the nose cone 7 and the first channel 21 and a third channel 23 communicating with the inside of the nose cone 7 and the fluid flowing path 14 and for flowing the seal medium between the rotor disc 8 and a blade root part 9a of each moving blade 9.