PURPOSE: To effectively execute an exposure operation by using a laser beam whose laser power is sufficient and to prevent that an optical element used to make the spectral line width of the laser beam narrow is deteriorated at an early stage by a method wherein a narrow-band laser beam and a non- narrow-band laser beam whose intensities have been controlled are combined.

CONSTITUTION: When an object 16 to be processed is exposed to light, a voltage which is applied to a first laser excitation part 3 and a second laser excitation part 4 is set by using an output control part 8. After that, individual laser oscillators 1, 2 are actuated; a narrow-band laser beam L1 and a non-narrow- band laser beam L2 are oscillated. The laser beams L1, L2 are reflected respectively by a first reflecting mirror 11 and a second reflecting mirror 13; they are incident on an intensity uniformization element 12 and are so combined as to make their intensity distribution uniform. A combined laser beam L is incident on a mask pattern 14; its image is formed by using an image formation lens 15; the object 16 to be processed is irradiated. The object to be processed can be exposed to light without lowering the processing efficiency; an element which makes the band of the laser beam narrow is not damaged at an early stage.