Control method and laser apparatus of a laser apparatus

A readily calculate the partial gas exchange of the laser gas in the laser chamber. A gas exchange step of exchanging said laser gain medium in the laser chamber for emitting a laser beam by exciting A laser gain medium, after the gas exchange, the predetermined gas pressure and predetermined in said laser chamber a first measuring step of measuring the pulse energy of the laser light is oscillated in the charging voltage of the predetermined gas pressure in the first measuring step, said predetermined charging voltage, based on the pulse energy, the laser chamber approximate expression of the relationship between pulse energy of the gas pressure and the charging voltage and the laser beam in, or the step of storing calculated a table indicating the correlation, after performing the first measurement step, in the laser chamber a second measuring step of measuring the pulse energy Er of oscillated laser beam, the calculated approximate expression Others on the basis of the table, said in the condition of the second measuring gas pressure and the charging voltage at the step, a step of calculating the pulse energy Eec that would have been obtained immediately after the gas exchange laser light, the pulse energy based on the the EEC pulse energy Er, ΔEd = from EEC-Er, calculating a decrease DerutaEd pulse energy, partial gas when performing partial gas exchange in the laser chamber based on the decrease amount DerutaEd of the pulse energy calculating a replacement amount Q, it may contain.