When a chamber of an oscillator and one or more amplifiers is to be replaced at the timing when a predetermined period elapses, one of the chambers of the oscillator and the amplifiers having a low allowable deterioration limit is detached and this chamber is attached in place of one of the chambers of the oscillator and the amplifiers other than the one having the low allowable deterioration limit. Thus, the chamber which has been used in one of the oscillator and amplifiers having a low allowable deterioration limit and has not reached the limit of deterioration is reused in one of the oscillator and the amplifiers other than the one having the low allowable deterioration limit. This enables efficient use of oscillator and amplifier chambers in a multistage amplification laser apparatus, minimizing the labor and parts consumed for replacement thereof.