The present invention is to provide a laser irradiation apparatus for forming a laser beam which has a shape required for the annealing and which has homogeneous energy distribution, by providing a slit at an image-formation position of a diffractive optical element, wherein the slit has a slit opening whose length is changeable.The laser irradiation apparatus comprises a laser oscillator, a diffractive optical element, and a slit, wherein the slit has a slit opening whose length in a major-axis direction thereof is changeable, wherein a laser beam is delivered obliquely to a substrate, and wherein the laser beam is a continuous wave solid-state, gas, or metal laser, or a pulsed laser with a repetition frequency of 10 MHz or more.