A light source unit used for a vehicular head-up display includes a prism-shaped multi-reflection member and a substrate. The multi-reflection member has: an entrance surface; an emission surface; and a reflective surface connecting the entrance surface and the emission surface. The substrate faces the entrance surface and is provided with a plurality of light sources mounted in a matrix. A ratio Ra=A/B or a ratio Rc=a/b is larger than a ratio Rb=C/D, where A and B is the number of the light sources provided in a longer and shorter direction of the entrance surface, respectively, C and D is a longer and shorter dimension of the emission surface, respectively, and a and b is the sum of lengths of light-emitting surfaces of the plurality of light sources in the longer and shorter direction of the entrance surface, respectively.