To provide a lens unit that achieves a sufficient wide-angle system and satisfies a demand for telecentricity.

The lens unit includes, in order from the object side to the image side, a first lens 11, a diaphragm 20, a second lens 12, and a third lens 13. The third lens 13 with a focal length f3 is convex and has positive refractive power. When the focal length of the entire optical system is f, the focal length of the first lens 11 is f1, and the focal length of the second lens 12 is f2, the lens unit satisfies (1)|f1/f|2, (2)|f2/f|2, and (3) 0.5f3/f3.