A vehicular lamp unit includes: a substrate on which a light source is mounted; a reflector including a substrate fixing portion and a reflector main body; and a screw. The substrate includes a first through hole, into which the screw is inserted; the substrate fixing portion includes a boss portion into which the screw is screwed; the reflector main body includes a base end portion fixed to the substrate fixing portion and a reflective surface to reflect light from the light source; the substrate is fixed to the substrate fixing portion in a state in which the substrate is held between the screw and the substrate fixing portion by screwing the screw, inserted into the first through hole, into the boss portion; and the substrate fixing portion includes at least one second through hole formed between the base end portion of the reflector main body and the boss portion.