[Problems]A laser beam irradiation apparatus which can accurately perform a linear welding with a uniform width on an irradiation portion even if the overlap ratio is lowered is provided.[Means for Solving the Problems]A laser beam irradiation apparatus includes laser beam generation means for emitting a laser beam, an optical fiber for transmitting the laser beam incident on an input side face to an output side face, an incident optical unit for introducing the laser beam emitted from the laser beam generation means to the input side face of the optical fiber, and an emission optical unit for applying the laser beam emitted from the output side face of the optical fiber to an irradiation portion, wherein the core cross section of the optical fiber is formed to be rectangular, preferably oblong, throughout the optical fiber or in a range at a predetermined distance from the output side face, and the length of the range where the core cross section is rectangular is preferably set to 3 m or above.