PURPOSE: To prevent the failure by heat diffusion by making the laser beam reflected by a rotating polygonal laser scanner to collimated beams of light, reflecting a part thereof, irradiating one side of an quadrilateral with this beam, reflecting the specific part of the transmitted light to both right and left sides and irradiating the remaining sides with the reflected light and the transmitted light.

CONSTITUTION: The light from a laser light source 10 is reflected by the rotating polygonal laser scanner 11. This laser beam is converted to the collimated beams of light by a collimator lens 12. The 1/4 the collimated beams of laser light are projected toward one side of the quadrilateral and the 3/4 the beams are transmitted. The laser beams are received by a 2nd beam splitter 14. The 1/3 the beams are transmitted through the splitter and the 2/3 the beams are reflected toward both the right and left sides. The transmitted and reflected laser beams are reflected by mirrors 15a, 15b, 15c toward the remaining three sides of the quadrilateral. The cream solder to the terminals, etc., arranged on the four sides of electronic parts is simultaneously melted in this way.