Vehicle-mounted light source unit

A vehicle-mounted light source unit (LU) is provided with: a heat sink (80) having a first rib (94L) and a second rib (94R); and a substrate (50) including a first through hole (51L) and a second through hole (51R) into which the first rib (94L) and the second rib (94R) are fitted, respectively. Each of the first rib (94L) and the second rib (94R) includes: an extending portion (94E) extending in a direction different from the left-right direction and having planar end surfaces; and a protruding section (94P) that protrudes from a portion of the extension section (94E) in the left-right direction toward the side where the first rib (94L) and the second rib (94R) are close to each other or away from each other, and the end surface of the protruding section (94P) has a planar shape having an in-plane direction different from that of both end surfaces of the extension section (94E). The first through-hole (51L) and the second through-hole (51R) each include a first pair of surface portions facing both end surfac