Lens unit

A lens unit, comprising:

• a holder having a housing space;
• a lens;
• a diaphragm means;
• and
• a cover member, wherein
• the lens, the diaphragm means and the cover member are housed individually in the housing space having a circular cross-section,
• side surfaces of the cover member are formed in a polygonal prism with even-numbered corners, the side surfaces continuing from a front end surface to a rear end surface which is located nearer to the lens,
• at least two of the corners of the polygonal prism are in contact with an inner peripheral surface of the housing space of the holder for housing the cover member,
• a gap is formed between each of the side surfaces of the cover member and the inner peripheral surface of the holder in the lens unit,
• an effective area of a light beam is located inside a light receiving area having a polygonal shape surrounded by a peripheral edge of the front end surface of the cover member,
• the outermost shape of the cross-sectional shape of the lens cut perpendicular to an optical axis direction is different from the outermost shape of the cross-sectional shape of the cover member cut perpendicular to the optical axis direction,
• a gap is formed between an outer periphery of the lens and an inner peripheral surface of the holder, and
• the outermost shape of the cross-sectional shape of the lens cut perpendicular to the optical axis direction is a circular shape.