A light irradiation apparatus which has excellent maintainability despite an increase in its length and size is provided. A light irradiation apparatus includes a plurality of light irradiation modules arranged; and a casing which houses at least part of the plurality of light irradiation modules. The plurality of light irradiation modules each includes a light irradiation device, a heat-dissipating member, a cooling piping for supply, a cooling piping for discharge which discharges a coolant from the flow channel, an electrical wiring, and a cover. The casing includes a side-cover and an under-cover.