An imaging unit (30; 30a) comprising: a columnar optical member (33; 38) having a substantially circular shape when viewed from a light incident direction, the optical member (33; 38) including an incident surface (33b; 38b) on which light is incident, a reflecting surface (33c; 38c) for reflecting the light incident from the incident surface (33b; 38b) in a direction different from the incident surface (33b; 38b), and an emission surface (33d; 38d) for causing the light incident from a direction orthogonal to the incident surface (33b; 38b) and reflected from the reflecting surface (33c; 38c) to travel in a straight line and emitting the light to the outside; an imaging device including a light receiving unit, formed on a surface of the imaging device, for receiving the light emitted from the emission surface (33d; 38d) and performing photoelectric conversion on the light; and a cylindrical imaging holder (31; 37) for defining the position of the incident surface (33b; 38b) and holding the optical member (33; 38) by making an inner peripheral surface of the imaging holder (31; 37) come into contact with a side surface of the optical member (33; 38), characterized in that the imaging holder (31; 37) includes an opening (31a; 37a) forming a cylindrical internal space in line with the maximum diameter of the incident surface (33b; 38b), the opening (31a; 37a) includes a cutout portion (31c; 37b) having a cut shape where part of a side surface of the cylindrical imaging holder (31; 37) is cut out and the imaging device is arranged at a position of the cutout portion (31c; 37b). a columnar optical member (33; 38) having a substantially circular shape when viewed from a light incident direction, the optical member (33; 38) including an incident surface (33b; 38b) on which light is incident, a reflecting surface (33c; 38c) for reflecting the light incident from the incident surface (33b; 38b) in a direction different from the incident surface (33b; 38b), and an emission surface (33d; 38d) for causing the light incident from a direction orthogonal to the incident surface (33b; 38b) and reflected from the reflecting surface (33c; 38c) to travel in a straight line and emitting the light to the outside; an incident surface (33b; 38b) on which light is incident, a reflecting surface (33c; 38c) for reflecting the light incident from the incident surface (33b; 38b) in a direction different from the incident surface (33b; 38b), and an emission surface (33d; 38d) for causing the light incident from a direction orthogonal to the incident surface (33b; 38b) and reflected from the reflecting surface (33c; 38c) to travel in a straight line and emitting the light to the outside; an imaging device including a light receiving unit, formed on a surface of the imaging device, for receiving the light emitted from the emission surface (33d; 38d) and performing photoelectric conversion on the light; and a cylindrical imaging holder (31; 37) for defining the position of the incident surface (33b; 38b) and holding the optical member (33; 38) by making an inner peripheral surface of the imaging holder (31; 37) come into contact with a side surface of the optical member (33; 38), characterized in that the imaging holder (31; 37) includes an opening (31a; 37a) forming a cylindrical internal space in line with the maximum diameter of the incident surface (33b; 38b), the opening (31a; 37a) includes a cutout portion (31c; 37b) having a cut shape where part of a side surface of the cylindrical imaging holder (31; 37) is cut out and the imaging device is arranged at a position of the cutout portion (31c; 37b).