A lamp unit includes a projection lens disposed on an optical axis, a light source disposed on a rear side of a rear focal point of the projection lens, a reflector configured to forwardly reflect light emitted from the light source toward the optical axis, and a shade configured to shield a part of the light reflected by the reflector. The lamp unit is configured to form a light distribution pattern having a cutoff line. An incidence plane of the projection lens includes a vertical diffusing portion which regulates the cutoff line. The vertical diffusing portion includes a plurality of lens elements extending substantially parallel to a horizontal portion of the upper edge of the shade. A vertical section of each of the lens elements is convex or concave with respect to a reference surface of the incidence plane.