To provide a crystallizing apparatus that stably form crystal grains of desired size by variably obtaining suitable dip strength corresponding to characteristics of an irradiated material without replacing an optical modulating element.
The light irradiation apparatus includes the optical modulating element (1) which has step lines of phase steps of phase differences substantially different from 180 and phase-modulates incident light, a lighting optical system (2) which lights up the optical modulating element with illumination light slanting to a direction nearly orthogonal to the step lines of the phase steps, and an imaging optical system (3) which forms a predetermined light intensity distribution on a predetermined surface to be crystallized based upon the light phase-modulated by the optical modulating element. The lighting optical system lights up the optical modulating element at the same time with first illumination light irradiating the optical modulating element in a first direction from a phase leading side to a phase lagging side of the phase steps and second illumination light irradiating the optical modulating element in a second direction from the phase lagging side to the phase leading side of the phase steps, and has a light intensity setting mechanism for setting the light intensity of the first illumination light and the light intensity of the second illumination light to substantially different values.