PURPOSE: To improve work efficiency by applying work with work width sufficiently expanded by splitting laser beam in TEMon mode while avoiding the center and then by inverting and composing split beams into one before irradiating a workpiece with them.

CONSTITUTION: Moving mechanism 7 moves triangular prism 5 and condenser lens 6 to a position coaxial, for example, to the optical axis of TEMon mode laser beam 3. As laser oscillator 1 emits laser light 3, workpiece 9 is irradiated with composite laser beam 10 with energy distribution shown in the figure. This laser beam 3 is wider in split-line-directional scan width than laser light obtained by splitting the original laser beam into two, for example, and then by inverting and composing split beams into one and, therefore, the work efficiency improves. Through the operation of mechanism 7, the peak position of the distribution can freely be changed, so that this device and method can be applied to a variety of work conditions.