PURPOSE: To improve the efficiency of energy utilization of beams by individually and independently freely selecting the polarizer of the respective beams and improving the intensity distribution of the beams in a reaction chamber.

CONSTITUTION: A laser source 12 delivers laser pulses along the beams S1 to Sn ((n) is an integer of ≥1) having selective excitation wavelengths λ1 to λn and the beam St of a wavelength λt capable of transforming the excited chemical species. The distribution apparatus is provided with a partial reflection mirror Mi ((i) is an integer of ≥1) which is arranged to superpose the transmitted and reflected beams S1 to Sn and St and guides the first part P1 of the beams to the reaction chamber 10 and guides the remaining part P2 to a subsequent partial reflection mirror and an optical delaying means 16 which is capable of synchronizing the pulses of the beam P2 guided from the mirror Mi to the subsequent mirror (Mi+l) and the pulses of the beam past the reaction chamber. Further, a returning means 14 adjusts the length of the reaction chamber pass stroke between the mirror Mi and the mirror (Mi+1).