A lens assembly includes: a lens group including a plurality of lenses; and a lens holder accommodating the lens group and having an opening in each of a front end face of the lens holder and a rear end face of the lens holder. The opening of the front end face has a smaller diameter than an outer diameter of a foremost lens accommodated in a foremost end face side of the lens holder, the lens holder includes a spacer ring disposed on a rear side of a rearmost lens accommodated in a rearmost end face side of the lens holder, the spacer ring abutting on a peripheral portion of the rearmost lens to keep a space between a rear face of the rearmost lens and the rear end face of the lens holder, and the lens holder holds a rear face of the spacer ring by thermal caulking.