In order to provide a line light irradiation device that can improve efficiency of condensing light with a compact size and that is almost free from unevenness of lighting, the line light irradiation device of the present claimed invention comprises multiple light emitting parts 2 each of which is provided with a light irradiating part 21 where multiple optical fibers 4 are thickly arranged in a line with light leading out end portions 4a of the multiple optical fibers 4 forming a straight line and a columnar lens 22 arranged to extend along a direction of the line P in front of the light irradiating part 21 in pairs, and that irradiate line light LL that converges into a straight line, and a holding body 3 that is arranged to face to a work W as being an object on which the line light LL is to be irradiated, on which monitoring bores 3a, 3b are arranged to penetrate in order to monitor the work W, and that holds the light emitting parts 2 so that each optical axis face of the line light LL irradiated from each of the light emitting parts 2 crosses on a predetermined straight line.