Abstract: A contact image sensor is provided including a housing, a slit plate a lens, one or two light sources and a light-receiving element array mounted on a light-receiving element array substrate. The housing contains the slit plate, the lens, the one or two light sources and the light-receiving element array substrate. The optical system of the contact image sensor is aligned and one or more depressions are formed on an end of the substrate for the alignment. Power to the one or two light sources is applied through one or more leads. Each of the one or more depressions is large enough so that each of the leads can be passed through the respective depressions.