An imaging unit includes an incident-side reflecting surface provided in a casing and reflects object-emanating light, entering the casing in a thickness direction thereof, along a lengthwise direction of the casing; a movable lens group movable along the lengthwise direction of the casing; an exit-side reflecting surface reflecting the light in the thickness direction of the casing; an image sensor provided in the casing and receiving the light from the exit-side reflecting surface; a light shield frame between the movable lens group and the exit-side reflecting surface; and a light-shield-frame position adjustment mechanism provided in the casing, which maintains a constant distance between the movable lens group an the light shield frame when the movable lens group moves in a first moving range, and varies the distance between the movable lens group and the light shield frame when the movable lens group moves in a second moving range.