Provided is a lens unit (1) in which a third lens (13) and a fourth lens (15) are press-fitted to a first cylindrical section (73) of a lens barrel (7) and the lens barrel (7) is fixed by a thread groove section (70) that is formed on the outer circumferential surface (73b) of the first cylindrical section (73). A circumferential groove (76) that is recessed toward the object side (L1) from an image-side end surface (730) between the inner circumferential surface (73a) of the first cylindrical section (73) and the outer circumferential surface (73b) is formed on the first cylindrical section (73) of the lens barrel (7). When viewed from a direction that is orthogonal to the optical axis (L), the bottom section (769) of the circumferential groove (76) is positioned more toward the object side (L1) than a press-fitted part (77) of the fourth lens (15) that is press-fitted to the inner circumferential surface (73a) of the first cylindrical section (73) nearest to the image side (L2).