A gas processing device that irradiates a processing target gas containing oxygen taken in from an air intake port with ultraviolet rays and then discharges the gas from an air exhaust port provided in a position different from a position of the air intake port, the gas processing device comprising: a casing including a first end including a first opening region forming the air intake port, a second end arranged in a position separated from the first end in a first direction, the second end including a second opening region forming the air exhaust port, and a main body portion having a hollow portion therein, the hollow portion communicating the first opening region and the second opening region; a discharge lamp arranged in the hollow portion and including a tube body having a shape extending in the first direction, a first electrode, and a second electrode, the discharge lamp that emits the ultraviolet rays from the tube body by application of a voltage between the first electrode and the second electrode; a first power supply line wired so as to pass through a side closer to the first end than the main body portion, the first power supply line that electrically connects the power supply unit to the first electrode; and a second power supply line wired so as to pass through the side closer to the first end than the main body portion, the second power supply line that electrically connects the power supply unit to the second electrode; and characterised in that, the gas processing device also comprises a power supply unit accommodated in a power supply box arranged outside the casing, and the power supply box being provided with a cooling air intake port, a cooling air exhaust port, and a cooling fan; and the gas processing device is configured such that outside air different from the processing target gas, taken into the power supply box from the cooling air intake port by operation of the cooling fan is discharged from the cooling air exhaust port after cooling the power supply unit. a casing including a first end including a first opening region forming the air intake port, a second end arranged in a position separated from the first end in a first direction, the second end including a second opening region forming the air exhaust port, and a main body portion having a hollow portion therein, the hollow portion communicating the first opening region and the second opening region; a discharge lamp arranged in the hollow portion and including a tube body having a shape extending in the first direction, a first electrode, and a second electrode, the discharge lamp that emits the ultraviolet rays from the tube body by application of a voltage between the first electrode and the second electrode; a first power supply line wired so as to pass through a side closer to the first end than the main body portion, the first power supply line that electrically connects the power supply unit to the first electrode; and a second power supply line wired so as to pass through the side closer to the first end than the main body portion, the second power supply line that electrically connects the power supply unit to the second electrode; and characterised in that, the gas processing device also comprises a power supply unit accommodated in a power supply box arranged outside the casing, and the power supply box being provided with a cooling air intake port, a cooling air exhaust port, and a cooling fan; and the gas processing device is configured such that outside air different from the processing target gas, taken into the power supply box from the cooling air intake port by operation of the cooling fan is discharged from the cooling air exhaust port after cooling the power supply unit.