Provided is a laser irradiation device for use in laser processing, and a laser processing method performed using the same. Provided is a laser irradiation device including: a light source that emits a laser beam; and an irradiation optical system which has one or a plurality of lenses, and is provided for light-guiding and light-converging of the laser beam emitted from the light source to the target substance, in which a birefringent material is used as a material entity of at least one lens of the irradiation optical system. Also provided is a laser processing method. The irradiation optical system may have a beam expander having a first lens that is a concave lens or a convex lens, and a second lens that is a convex lens, in the order along the laser-beam travel direction, the beam expander being configured so as to make the interval between the first lens and the second lens variable, in which a birefringent material may be used as a material entity of the first lens and/or the second lens. Thus, the interval between the lenses can be changed by a convenient operation, whereby relative positions between the plurality of beam waists (interval) can be easily regulated.