To provide a method and a device for improving quality in laser beam machining by reducing laser beam leakage into a processed head on non-processed side.

The device uses a polarizer 14 to divide the laser beams outputted from one laser oscillator 10 and time-shared by EOM12 for each polarized direction, and delivers them to a plurality of processed heads 18 and 20. In this case, additional polarizers 30 and 32 reduce the laser beam leakage into the processed head on non-processed side by further subdividing the branched laser beams.