A lens unit according to the present invention includes: a barrel, which has first and second openings and an inner space defined along a center axis between the first and second openings; and lenses held in the inner space. In each pair of adjacent lenses, when projected parallel to their optical axis, one lens located closer to the second opening has a greater projection than the other lens located closer to the first opening. The barrel has inner side surface portions, which are arranged parallel to the center axis to define cross sections corresponding to the respective projections of the lenses. In each pair of adjacent inner side surface portions, one portion located closer to the second opening has a greater cross section than the other portion located closer to the first opening. The barrel has main grooves cut on the inner side surface portions to run from the first opening toward the second opening. Each of the lens units is bonded to an associated inner side surface portion with an adhesive that fills at least partially the main grooves on the inner side surface portion.