Provided is a light irradiation device capable of appropriately suppressing positional misalignment between optical axes even though a plurality of light source modules are disposed in series. A light irradiation device includes a plurality of light source modules disposed in series, in which the light source modules each include a substrate, LED elements disposed on a main surface of the substrate, and an optical element having sidewall sections installed uprightly on side sections of the substrate, and a lens section supported by the sidewall sections and positioned above the LED elements, in which the optical element is made by integrally forming the sidewall sections and the lens section from the same material, and in which a linear expansion coefficient of the substrate, a linear expansion coefficient of the optical element, and a difference in linear expansion coefficient between the substrate and the optical element are within specific ranges.