PURPOSE: To enhance efficiency of photochemical reaction for output of laser by regulating the focal length and angle of a second light reflector and a third light reflector so that the positions of focus coincide in the respective depth of a focus in the direction of optical axis and also are superposed at two or more times in the direction vertical to optical axis while being successively shifted.

CONSTITUTION: Laser beams 1 start to be converged by a condenser lens 2 and are made incident from a window member of the incident end side of a reactor 5 through which a gaseous raw material 6 flows. This gaseous raw material 6 is introduced into the reactor 5 from the incident end side of the laser beams 1 and flows coaxially to the optical axis of the laser beams 1, and is subjected to photochemical reaction and thereafter discharged together with a reaction product from the reverse side of the reactor 5. Laser beams 1 are passed through the lens 2 and focus in the reactor 5 and thereafter are reflected by a spherical mirror 3. The laser beams 1 refocus in the reactor 5 and thereafter reach a spherical mirror 4. Multiple reflections are repeated between two sheets of spherical mirrors 3, 4 and superposition is realized in the reactor 5.