A lamp assembly is provided, that utilizes a light source including an LED element without cutting part of light therefrom and capable of forming a luminance distribution where the light with a maximum peak portion can be arranged substantially at the cutoff line. The lamp assembly used for a vehicle can include: a light source including an LED element and a wavelength conversion layer configured to cover an emission surface of the LED element and emit white light formed by additive color mixture; and a projection optical system for projecting an image of the light source in the illumination direction so that a desired light distribution pattern can be formed on a virtual vertical screen. The LED element has a rectangular shape having long sides and short sides, and can be configured to provide a luminance peak portion at or near one of the long sides, then form a cutoff line of the desired light distribution pattern by utilizing the luminance peak portion of the light source image.