PURPOSE: To provide an excimer laser apparatus in which gas medium for laser is purified and circulated during operation, and an operating method therefor by which stable laser output is easily obtained for a long time.

CONSTITUTION: Gas lines A and B are connected to the laser chamber of an excimer laser in parallel in order to remove impurities produced in laser gas media. The gas line A includes a cold trap 2, and the gas line B includes a low temperature absorption tower 4 filled with cooled active carbon absorbent. In the case of KrF gas laser, SiF4 and HF are efficiently removed by the cold trap 2 in the line A, and CF4, unremovable by the trap, is removed in the low temperature absorption tower 4 in the line B. Controlled use of the line including the absorption tower reduces the consumption of required gas components, and extends the life of a gas purification system.