The present invention provides a light guide, a line-illuminating device, and an image-scanning device that provide excellent uniformity in the amount of light produced. A light-reflecting portion comprised of spherical concave surfaces becomes a reflecting surface having spherical convex surfaces when it is seen from the inside of the light guide. Even when the incident angle of light from the light source unit is slightly offset, the reflection angle in spherical convex surfaces significantly changes, and light is reflected uniformly toward the upper surface serving as a light-emitting surface. In particular, when the fine spherical concave portions of the light-reflecting portion have a small depth, the incident light from the end surface is reflected toward the upper surface, serving as a light-emitting surface, and both side surfaces without reflecting back to the light-emitting unit side, so that uniform reflection can be achieved.