A light illuminating apparatus used for a laser annealing apparatus includes a first light splitting unit (16) and a second light splitting unit (20) for splitting a sole laser beam into n laser beams, and a synthesis unit (21) for synthesizing an m'th laser beam, radiated from the first light splitting unit (16), and an m'th laser beam radiated from the second light splitting unit (20), to each other, where m is an integer not less than 1 and not larger than n. The first light splitting unit (16) and the second light splitting unit (20) are formed by the same optical components and are arrayed inverted with respect to each other.