To provide an excimer laser device and method in which the frequency of
full gas exchange within the laser chamber is reduced, and more
preferably full gas exchange is made unnecessary.The gas supply device
and gas exhaust device are controlled so that the laser gas in the laser
chamber is partially exchanged in a gas exchange quantity that maintains
the quantity of impurities in the laser chamber at or below a fixed
level. Also, the gas exchange quantity is obtained using the total
quantity of output light energy reduction A, the total gas pressure in
the laser chamber P, and output light energy reduction quantity per unit
time k, for the case where partial gas exchange is repeated infinitely in
the laser chamber.