The present invention is a laser irradiation apparatus which is capable of selecting a laser light of a wavelength among a plurality of laser lights having different wavelengths according to a purpose of irradiation and irradiating an object to be irradiated with the laser light having the wavelength. The laser irradiation apparatus includes, on an optical path of a mixed light beam of the plurality of laser lights having different wavelengths, a slit having a width corresponding to a diffraction limit of the laser light having the shortest wavelength among the plurality of laser lights having different wavelengths, and an objective lens for focusing a laser light that has passed through the slit on the object to be irradiated.