An image sensor includes an elongated case, a light source provided in the case, a light guiding member elongated in a longitudinal direction of the case and configured to direct light emitted from the light source toward a document to be read, an optical system for collecting light reflected by the document, a sensor IC chip for receiving the light collected by the optical system, and a substrate attached to the case. The substrate is provided with a wiring pattern and carries the sensor IC chip. The case is provided with a plurality of supporting projections spaced apart from each other in the longitudinal direction. The supporting projections are held in contact with the side surface of the light guiding member.