A light irradiation device includes a protective tube, which has a wire insertion path therein, a light source unit facing and disposed along an upper part of the protective tube, and a gutter-shaped concave reflection mirror facing the light source unit and provided below the protective tube. The concave reflection mirror is received in a gutter-shaped concave accommodating part provided in a holding body, and has flange portions extending from the outer surface in the horizontal direction. The convex reflection mirror is detachably affixed to the holding body with the flange portions.