A line-illuminating device which is resistant to shrinkage problems caused by repetition of heating and cooling includes a white casing made of polycarbonate or the like, a light guide made of an acrylic resin or the like and accommodated in the casing such that the light-emitting surface thereof is exposed, and light-emitting elements (for example, light-emitting diodes) as a light source provided on both ends of the casing so as to abut against the end surfaces of the light guide without a gap. The casing is divided into two portions in the longitudinal direction, and a gap is formed between the two divided portions. With this gap, even if the light guide shrinks due to repetition of heating and cooling, the divided portions can effectively shrink together with the light guide via the gap, and the abutting state between the end surface of the light guide and the light-emitting element can be maintained.