PURPOSE: To let a pulse cycle to reach a target cycle and use laser output more efficiently by forcing two pulse light to enter the same beam splitter from each opposite side and lapping one reflected by pulse light with the other permeated by pulse light.

CONSTITUTION: Two excitation optical source CVL 1a and 1b are used where a dye laser oscillator 2 is employed as a laser oscillator excited by the CVL 1a and 1b and a dye laser amplifier 3 is also used as an amplifier. The optical sources CVL 1a and 1b are designed to operate with the highest efficiency during its repeated oscillation operation. Either optical source operates based on a signal from a pulse controller 4 which includes a trigger circuit designed for oscillation. The CVL 1a and 1b are laid out in such a position that their respective emitted light may cross each other while a beam splitter 6a is also laid out therein. In addition, the amount of reflection and the amount of permeation are predetermined so as to meet their target values. Beam splitters 6b and 6c and mirrors 7a, 7b, and 7c are laid out at a position where the light can be entered at a specified rate.