An imaging unit includes a light source placed on one surface of a light transmissive plate-like element to project a light to the one surface of the plate-like element, an imaging element to capture an image of an attached matter on the other surface of the plate-like element illuminated with the light from the light source, and an optical element having an incidence surface on which the light is incident from the light source, a reflective surface by which the light incident from the incidence surface is reflected, a transmissive surface contacting the one surface of the plate-like element, through which the light reflected by the reflective surface transmits, and an exit surface from which the light transmitting through the transmissive surface and reflected by the other surface of the plate-like element is emitted towards the imaging element.