PURPOSE: To provide an excimer laser gas refining method by which all the impurities such as CF4, N2 and O2 produced in laser gas are completely removed and maintain the laser output stably for a long time.

CONSTITUTION: A heavy metal added active carbon trap is employed as an F2 removing trap 4 and a zirconium system alloy trap whose temperature is controlled at 420°C is employed as an air component removing trap 5. After laser gas in an excimer laser apparatus 1 is circulated through the respective traps by a circulating pump 2, the laser gas is returned to the excimer laser apparatus 1. It is noted that F2 which is removed by the F2 removing trap 4 is re-added by an F2 adder 6. As all the impurities including CF4, N2, O2, etc., which are produced in the laser gas and cause the decline of the laser output can be completely removed, the laser output can be maintained stably for a long time.