An imaging unit includes a housing having an opening on one of opposite sides in a direction of thickness thereof; an imaging optical system provided in the housing and includes an incident surface and an exit-side prism, wherein object-emanated light being incident on said incident surface and traveling in a lengthwise direction of said housing is reflected by said exit-side prism toward said opening; an image sensor, installed in the housing, with an imaging surface facing the other of the opposite sides of the housing toward an exit surface of the exit-side prism; a spacer projecting from the housing or a fixed member, the spacer contacting the image sensor to create a clearance between the exit surface and the imaging surface; and a cover fixed to the housing to close the opening and press the image sensor so as to come in contact with the spacer.