To minimize the dispersion in exposure amount, by a method wherein discharge voltage is controlled per pulse from starting pulse oscillation, so as to suppress the energy fluctuation in laser beams caused from the starting time of the pulse oscillation.

A pattern of discharge voltage for the pulse oscillation time so as to specify the pulse energy of laser beams is stored in a computer 4 as data. This pattern is decided by the relation between the discharge voltage and the energy of laser beams when the least pulse numbers are oscillated in the stoppage state of burst mode. Besides, a laser part 7 receiving an oscillation order controls a power supply according to the pattern data memorized at that time for the pulse oscillation while changing the discharge voltage with time. Through these procedures, the laser beams at a constant pulse energy level can be emitted immediately after the oscillation of the burst mode, thereby enabling the dispersion in exposure amount to be minimized even in the least pulse numbers.