A line light irradiation device includes a plurality of light source units on which a plurality of light source mounting substrates are juxtaposed on a base in a row, and the plurality of light source units are juxtaposed in a row. The plurality of light source mounting substrates are positioned and fixed to the base at predetermined intervals. Thus, if a light source in a certain light source unit becomes defective, this light source unit can be removed so as to exchange only the light source mounting substrate on which the defective light source is mounted. Therefore, the normal light sources can be left as they are as much as possible, and waste of the light sources can be reduced. Further, the light source mounting substrates are positioned and fixed at predetermined intervals, and thus dimensional errors do not accumulate in the light source mounting substrates at the juxtaposition ends.