Laser annealing device (1) irradiates amorphous silicon film (W1) with laser beam (Li) to perform an annealing process. The laser annealing device includes: a plurality of laser oscillators (2i) that emit laser beams having mutually different wavelengths (λi); diffraction grating (3) that diffracts the laser beams emitted from the laser oscillators; and controller (6) that switches on and off states of emission of the laser beams by the laser oscillators. The laser oscillators are disposed at mutually different positions, and the laser beams emitted from the laser oscillators are diffracted on identical optical axis (A) by the diffraction grating. The controller can select at least one or more of the laser oscillators for turning on emission of the laser beams from among the plurality of laser oscillators in accordance with any crystal grain size of the amorphous silicon film.